New message flagging code.

New message flagging code.

Framework is in place to allow user-defined
flags/keywords/labels/whatever you want to call them. All code is
present except for actual viewing of the user-defined flags. Still
working out the UI - I think it is going to end up looking like the
message labeling as seen in Gmail - trying to allow custom icons is way
beyond the scope of this feature. But the flags are correctly set on
the messages.

This implements (most) of Ticket #937.