Appearance Prefs: drag and drop between ColorWhichItems

Fixes final piece of #8618

Already added support for list items to drag colors out and you can
drag and drop between the list items and preview. but, what was
missing was drag and drop between list items. Updated
ColorWhichListItem to also accept color drops through their
parent ColorWhichListView.

Also included some related style fixes, use B_RGB_COLOR_TYPE
constant in place of (type_code)'RGBC'. 80-char limit fixes.

Simplify similar code in ColorPreview class to parse out rgb_color
from message.

ColorPreview passes dropped color along to APRView

APRView no longer accepts color drops, this is handled by ListView
and ColorPreview now.

Consolidated "RGBColor" and "which" message name strings into
constants defined in defs.h.

Change-Id: I88ec2a4ffe077620ec4cc3b032196cbff0f09615

Appearance: Another BString::SetToFormat instead of sprintf.

Appearance: Make color which items draggable

Create a ColorWhichListView class which impliments InitiateDrag()
to drag a color square out of the list view. You can then drop the
color anywhere that accepts a color drop. This allows you to drag
and drop colors between list items for example.