FakeVim: Use QTextDocument::availableUndoSteps() for undo/redo

Partially reverts commit b9cc16e405378a927cf013ef0b89c964532aa03a.

Using QTextDocument::revision() to keep track which undo/redo command
should be used next doesn't help much and it's more difficult to use
than QTextDocument::availableUndoSteps().

Task-number: QTCREATORBUG-9784
Change-Id: I656e1e964477de0f387c80b9384a32e0d8dab39f
Reviewed-by: hjk <hjk121@nokiamail.com>
FakeVim: Fix checking if text was re-inserted

Change-Id: I213ad5c9d6f8d313f07daaceb253a7d03b9035a3
Reviewed-by: hjk <hjk121@nokiamail.com>