Refactor Token and DropTarget

Add lots of comments.
Reorder Token constructor according to scope
Get rid of Token.m_textColor and use palette instead of constly html
format.
Change Margins, expanding and spacers so that the drop target is
center aligned in the OrganizeCollection dialog.
Set minimum size to drop target so that an empty drop target
has enough space for the "drop here" text.
Clean up DropTarget