In paint bucket mode the user can undo by right-clicking.
Undoing, however, is only possible after the first paint op.
After undoing is enabled, the user can undo indefinetely by
righ-clicking. This is not consistent and can be confusing.

This patch tracks the changes done after entering the paint
bucket mode, and allows the user to undo them using right-clicking
until they reach the first operation before entering this context.

Because the user can undo/redo using other means, the patch takes
it into account and will reset its tracking to accomodate this.

Ultimately, the user is able to make any changes in paint bucket
mode and undo them all by right-clicking, but no further than that.

Change-Id: I93380fb60db83aee37a1d324218ba6ab1700a2e5
Reviewed-on: https://gerrit.libreoffice.org/16951Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>