"Select color from anywhere on screen" added to color selector

See #104.

Many thanks to LaVolpe, who first prototyped a feature like years ago
(http://www.planetsourcecode.com/vb/scripts/ShowCode.asp?txtCodeId=52878&lngWId=1).
(Warning: his implementation is cryptic and deeply buried, as usual!)  I
drew many ideas from his work, which uses some clever workarounds to
allow mouse capture without a button being pressed.

Note that this feature is a bit finicky in the IDE, due to inevitable
performance issues while redrawing the entire dialog as the mouse moves
around the screen.  Compile for best results.

Also added the ability to revert to the original color by clicking the
"original" color box.