Copy/Paste property paths/values in inspector.

Resolves godotengine/godot-proposals#106.

Adds the following property menu options with default bindings:

- Copy Property (ctrl+c)
- Paste Property (ctrl+v)
- Copy Property Path (ctrl+shift+c)

If you hover over a property label in the inspector dock, you can copy
either the property value or the property path to the system clipboard
using the shortcuts above This is especially useful for the
`AnimationTree`, where code might reference properties like
"parameters/state/aim/move/blend_position".

One issue is that if you click a property, then click on the node you
currently have selected in the node tree, then press ctrl+shift+c, it
will still copy the selected property path rather than the node path. If
you click on a different node in the nodetree, however, ctrl+shift+c
will return to copying the nodepath.

The property value copy/paste was implemented by @KoBeWi at #39398 and
merged into this PR due to their similarity.

editor/editor_inspector.cpp

@@ -31,11 +31,13 @@
 #include "editor_inspector.h"
 
 #include "array_property_edit.h"
+#include "core/os/keyboard.h"
 #include "dictionary_property_edit.h"
 #include "editor/doc_tools.h"
 #include "editor_feature_profile.h"
 #include "editor_node.h"
 #include "editor_scale.h"
+#include "editor_settings.h"
 #include "multi_node_edit.h"
 #include "scene/resources/packed_scene.h"
 
@@ -782,6 +784,30 @@ void EditorProperty::gui_input(const Ref<InputEvent> &p_event) {
 			update();
 			emit_signal(SNAME("property_checked"), property, checked);
 		}
+	} else if (mb.is_valid() && mb->is_pressed() && mb->get_button_index() == MOUSE_BUTTON_RIGHT) {
+		_ensure_popup();
+		menu->set_position(get_screen_position() + get_local_mouse_position());
+		menu->set_size(Vector2(1, 1));
+		menu->popup();
+		select();
+		return;
+	}
+}
+
+void EditorProperty::unhandled_key_input(const Ref<InputEvent> &p_event) {
+	if (!selected) {
+		return;
+	}
+
+	if (ED_IS_SHORTCUT("property_editor/copy_property", p_event)) {
+		menu_option(MENU_COPY_PROPERTY);
+		accept_event();
+	} else if (ED_IS_SHORTCUT("property_editor/paste_property", p_event) && !is_read_only()) {
+		menu_option(MENU_PASTE_PROPERTY);
+		accept_event();
+	} else if (ED_IS_SHORTCUT("property_editor/copy_property_path", p_event)) {
+		menu_option(MENU_COPY_PROPERTY_PATH);
+		accept_event();
 	}
 }
 
@@ -895,6 +921,20 @@ String EditorProperty::get_tooltip_text() const {
 	return tooltip_text;
 }
 
+void EditorProperty::menu_option(int p_option) {
+	switch (p_option) {
+		case MENU_COPY_PROPERTY: {
+			EditorNode::get_singleton()->get_inspector()->set_property_clipboard(object->get(property));
+		} break;
+		case MENU_PASTE_PROPERTY: {
+			emit_changed(property, EditorNode::get_singleton()->get_inspector()->get_property_clipboard());
+		} break;
+		case MENU_COPY_PROPERTY_PATH: {
+			DisplayServer::get_singleton()->clipboard_set(property);
+		} break;
+	}
+}
+
 void EditorProperty::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_label", "text"), &EditorProperty::set_label);
 	ClassDB::bind_method(D_METHOD("get_label"), &EditorProperty::get_label);
@@ -971,6 +1011,21 @@ EditorProperty::EditorProperty() {
 	label_reference = nullptr;
 	bottom_editor = nullptr;
 	delete_hover = false;
+	menu = nullptr;
+	set_process_unhandled_key_input(true);
+}
+
+void EditorProperty::_ensure_popup() {
+	if (menu) {
+		return;
+	}
+	menu = memnew(PopupMenu);
+	menu->add_shortcut(ED_GET_SHORTCUT("property_editor/copy_property"), MENU_COPY_PROPERTY);
+	menu->add_shortcut(ED_GET_SHORTCUT("property_editor/paste_property"), MENU_PASTE_PROPERTY);
+	menu->add_shortcut(ED_GET_SHORTCUT("property_editor/copy_property_path"), MENU_COPY_PROPERTY_PATH);
+	menu->connect("id_pressed", callable_mp(this, &EditorProperty::menu_option));
+	menu->set_item_disabled(MENU_PASTE_PROPERTY, is_read_only());
+	add_child(menu);
 }
 
 ////////////////////////////////////////////////
@@ -2615,6 +2670,14 @@ void EditorInspector::set_restrict_to_basic_settings(bool p_restrict) {
 	update_tree();
 }
 
+void EditorInspector::set_property_clipboard(const Variant &p_value) {
+	property_clipboard = p_value;
+}
+
+Variant EditorInspector::get_property_clipboard() const {
+	return property_clipboard;
+}
+
 void EditorInspector::_bind_methods() {
 	ClassDB::bind_method("_edit_request_change", &EditorInspector::_edit_request_change);
 
@@ -2657,6 +2720,7 @@ EditorInspector::EditorInspector() {
 	property_focusable = -1;
 	sub_inspector = false;
 	deletable_properties = false;
+	property_clipboard = Variant();
 
 	get_v_scrollbar()->connect("value_changed", callable_mp(this, &EditorInspector::_vscroll_changed));
 	update_scroll_request = -1;
@@ -2666,4 +2730,8 @@ EditorInspector::EditorInspector() {
 		//used when class is created by the docgen to dump default values of everything bindable, editorsettings may not be created
 		refresh_countdown = 0.33;
 	}
+
+	ED_SHORTCUT("property_editor/copy_property", TTR("Copy Property"), KEY_MASK_CMD | KEY_C);
+	ED_SHORTCUT("property_editor/paste_property", TTR("Paste Property"), KEY_MASK_CMD | KEY_V);
+	ED_SHORTCUT("property_editor/copy_property_path", TTR("Copy Property Path"), KEY_MASK_CMD | KEY_MASK_SHIFT | KEY_C);
 }