Disable multi-window buttons instead of hiding them when support is unavailable

This is more explicit as for why this functionality isn't available
depending on editor settings and current platform.

This also exposes a `EditorInterface.is_multi_window_enabled()` method
so that editor plugins can easily query whether the editor is able and
expected to create multiple windows.

doc/classes/EditorInterface.xml

@@ -195,6 +195,15 @@
 				Shows the given property on the given [param object] in the editor's Inspector dock. If [param inspector_only] is [code]true[/code], plugins will not attempt to edit [param object].
 			</description>
 		</method>
+		<method name="is_multi_window_enabled" qualifiers="const">
+			<return type="bool" />
+			<description>
+				Returns [code]true[/code] if multiple window support is enabled in the editor. Multiple window support is enabled if [i]all[/i] of these statements are true:
+				- [member EditorSettings.interface/multi_window/enable] is [code]true[/code].
+				- [member EditorSettings.interface/editor/single_window_mode] is [code]false[/code].
+				- [member Viewport.gui_embed_subwindows] is [code]false[/code]. This is forced to [code]true[/code] on platforms that don't support multiple windows such as Web, or when the [code]--single-window[/code] [url=$DOCS_URL/tutorials/editor/command_line_tutorial.html]command line argument[/url] is used.
+			</description>
+		</method>
 		<method name="is_playing_scene" qualifiers="const">
 			<return type="bool" />
 			<description>