Add "color selection" ability to preview windows

This feature can optionally be enabled by setting the preview control's
.AllowColorSelection property.  When enabled, the user can click the
preview window to raise a ColorSelected event, which the parent form can
then handle as desired.  (Typically, by reading out the selected color
and applying it to the preview.)

This commit also fixes a bug where 32bpp images were not properly
composited when "show original image" was selected.

Behavior is not ideal; I'm waiting on a fix from Kroc that addresses
MouseMove events in his mouse tracking class.  Once that arrives, I'll
improve the original/preview swapping behavior while the preview control
has focus.
Enable preview color selection on relevant tool dialogs