Fix crashes on undo/redo of vector path points operations

1) KoPathToolSelection keeps pointers to the shapes and its path points,
   therefore it should subscribe to the updates on the shapes and their
   points (using ShapeChangeListener framework).

2) The patch also fixes a lot of bugs in "Join by segment" and
   "Merge points", which should behave correctly now.

3) The patch also partially implements "keep path point selection"
   idea, which selects newly created point after each operation. The
   stuff is achieved using KoPathShape::PointSelectionChangeListener
   listener.

BUG:393209
BUG:393087