Basic implementation of saving Mirror Tool state to KRA file, wished for in bug 339515. It is working, though there is still some work to be
done and bugs to fix (toolbar buttons not toggled correctly upon document change, etc.). The implementation is inspired
by other canvas decorations (grids, guides, etc.). Before I continue further, I would like to ascertain whether this is
the right direction, or whether I should do things differently.
BUG:339515