Add disabled undo & redo buttons for when there is nothing to undo/redo

[SamTheBlow]

When there is nothing to undo, the undo button grays out. When there is nothing to redo, the redo button grays out.
The buttons are updated correctly when creating a new file or when going to a different tab.

This is only a visual change. Hovering over the buttons still shows a hint, and clicking on them still makes them do their thing.

Note: this does not behave as expected when the user zooms in/out. For some reason, zooming in/out marks a project as "dirty", which enables the undo button as a result. I've decided to ignore this because zooming in/out shouldn't mark a project as "dirty" in the first place (please correct me if I'm wrong)

Also, I know this project is currently being updated to Godot 4: I don't mind waiting and rewriting the PR after that's done.
(This is kinda my first PR, sorry if I screwed something up)

[mbrlabs]

Hello, sorry for the late reply. I finally ported Lorien to Godot 4. I'd like to merge your change, so if you're still interested feel free to update the PR :)

[SamTheBlow]

Nice! Working on it asap.