TextInput: Add undo/redo support

[Waqar144]

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#474

[ogoffart]

Olivier, do you have any preferences here?

I prefer using SharedVector

[tronical]

@Waqar144 Would you like me to take over to take care of the binding bits, or would you like to give it a try?

[Waqar144]

I had to change things quite a bit to make it work:

• added a pop() method to SharedVector
• UndoItem is now a struct with a UndoItemKind field, previously it was an enum. I had to change it because the old struct was non-trivial