Remove all old specialty Selection Undo/Redo code

Really nice to have that old crap gone!  Selections are now more
responsive, and they'll only get faster as we continue work on the new
Undo/Redo engine.

Some selection settings still need to be updated to work with Undo/Redo,
in particular any settings on the selection toolbox (e.g. the feathering
slider).  By design, these UCs do not automatically trigger undo/redo
creation, so if the user creates a selection, then modifies its
feathering radius, then applies an effect, the Undo stack will jump
straight from "create selection" to "apply effect".

This is a tough problem to solve, because PD's various custom user
controls only raise a single "Change" event (again, by design), and we
don't want to create Undo data for every slide of the bar - just when
the mouse is released.  Similarly, there is probably wisdom to
condensing multiple use of those toolbox sliders into a single Undo/Redo
entry, rather than adding every unique interaction to the stack.

The solution I'm considering is to check Selection status before
initiating any action in the main Process function.  If a selection is
currently active, the sub will compare the current selection state to
the last known selection state in the Undo/Redo stack.  If the two
differ, a new Selection-based Undo/Redo entry will be submitted
immediately, thus encompassing any Selection changes between the
previous Undo creation and the present one.  In theory, this should
solve our problem, without requiring too much specialty Undo/Redo code,
and if it works well, we can use it as a model for other "on-canvas"
adjustments in the future.