Undo/redo bug fixes

[J5lx]

CandyFace mentioned that #953 probably won’t get merged very soon, so I went ahead and killed a few bugs in the current undo implementation. The fixes are simple enough that it’s hard to justify letting our users wait for the rewrite.

• Fixed broken smudge undo (Smudge Tool bug: Can't undo smudges. #1004)
• Fixed weird undo/redo behaviour for selection flipping (Selection Flipping is not undoable / redoable #1098, Undoing Selection Flip X / Y will delete the drawing #1120) by adding explicit support for it
• Crash when undoing after deleting a layer (Crash when undoing after layer deletion #1412)
  • The code for this is kinda long, but mostly just because its inflated with boilerplate code
  • The actual layer deletion undo support (Allow to UNDO / REDO "deleting" new frames and layers #748) still has to wait for the rewrite, though
• Unwanted keyframes added when undoing strokes drawn on an unselected keyframe through an empty frame (Undoing strokes with specific timeline drawing behavior creates unwanted keyframes #1429)

[MrStevns]

LGTM!

I tested the mentioned bugs and they all seem to be fixed.
Well done 👍