* 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.
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