Improve Undo/Redo menu items

* Make Undo/Redo menu items disabled when clicking it does nothing.
    * Context menu of `TextEdit`
    * Context menu of `LineEdit`
    * Editor's Scene menu
    * Script editor's Edit menu and context menu (for Script and Text)
* Make editor undo/redo log messages translatable.
* Mark `UndoRedo`'s `has_{un,re}do()` methods as `const`.
* Expose `TextEdit`'s `has_{un,re}do()` to scripts since `{un,re}do()` are already available.

doc/classes/TextEdit.xml

@@ -464,12 +464,24 @@
 				Returns if the user has IME text.
 			</description>
 		</method>
+		<method name="has_redo" qualifiers="const">
+			<return type="bool" />
+			<description>
+				Returns [code]true[/code] if a "redo" action is available.
+			</description>
+		</method>
 		<method name="has_selection" qualifiers="const">
 			<return type="bool" />
 			<description>
 				Returns [code]true[/code] if the user has selected text.
 			</description>
 		</method>
+		<method name="has_undo" qualifiers="const">
+			<return type="bool" />
+			<description>
+				Returns [code]true[/code] if an "undo" action is available.
+			</description>
+		</method>
 		<method name="insert_line_at">
 			<return type="void" />
 			<argument index="0" name="line" type="int" />