Central Processor: fix Undo behavior when resizing/rotating image and a selection is active

In a perfect world, any image transformations (resize, rotate, etc),
would also transform the active selection.  Unfortunately, we do not
live in a perfect world, and I have a lot of other things I'd rather
work on than making every selection type transformable this way.

What PD has historically done for actions that change image size, is
check to see if a selection is active, and remove prior to proceeding.
I recently discovered that this creates problems if the user uses Undo
to return to that point in time, because the Undo/Redo engine was never
notified of the selection removal, and thus it thinks a selection is
still active at that point in time.  This causes the selection object
and image state to fall out of sync, potentially causing crashes.

This commit uses a new approach, where the central processor actually
checks for image transformations, and if one is found, a formal "Remove
selection" process request is made before proceeding.  This keeps the
Undo/Redo engine in sync with both image and selection state, and it
gives the user a way to recover a selection that has been automatically
removed due to a resize or rotate event.