New GDI+ fallback for selection feathering (blurring)

Way faster than my homebrew routine.  v1.1 of GDI+ is required.  Note
that v1.1 is included by default on Vista/7/8, but not on XP.  It may
exist on XP from external MS software installs or possibly service packs
(not certain).

If GDI+ 1.1 isn't found, the software will fall back to its internal
Gaussian Blur routine, which is significantly slower.