Selection engine: more refactoring, bug-fixes

- Git should ignore my local /Data/Selections folder
- All selection save/load functions now branch according to selection
type.  This will make it a breeze to add Undo/Redo support for new
selection types.
- Fixed a bug where selections dragged or resized completely off the
image were still being allowed.  These must be disallowed, or they will
cause OOB errors if a tool dialog is initiated.
- Removed a number of dead functions that were not relevant since the
last Undo/Redo engine overhaul.
- Made a number of functions more generic, so they will automatically
operate on new selection tools in the future.
- Simplified a number of overly complicated functions.
- Removed a lot of code that was duplicating identical behavior between
different selection types
Selection engine: move all Select menu items out of pdSelection...

...and into the Selection_Handler module, where they make much more
sense.

Also, all Select menu items (grow, shrink, feather, etc) have been
optimized against new PD toolkits.  The quickBlurDIB() function was
particularly useful here, so all those menu items should perform
significantly better now.  This will be relevant as we add selection
types that actually make use of the menu items (e.g. magic wand
selections, which are not vector-based, so they cannot automatically be
of "Border" type).