Applies all undo items, without adding redo items. The initial text (before any modification has been done since the undo stack is enabled) will be recovered. The undo stack is empty afterwards.

Note: If edit irreversible returns true (before calling this function), then it's not possible to recover the initial text, in this case it will be recovered as much as possible. If edit irreversible returns false (before calling this function), then the state edit altered will change to false.