Bug 719634 - Palette editor not working for New Palettes (cannot drop colors)

Allow to drop colors also on the palette view's parent viewport, so
colors can be dropped everywhere inside the scrolled window, also when
the palette view is invisible because it has zero colors. Also allow
dropping of palettes on the viewport to change the editor's active
palette.

(cherry picked from commit 0e1978d27346c6ba952945d48c50c0bb64ec1ca8)