Party Bar

The player party bar has now been reimplemented and is fully featured.

Portrait windows (of which there's one for each party member, up to 4)
now consist of three elements; an outer frame, an inner frame and a
portrait. The outer frame responds to the mouse cursor, changing colour
when hovered over or clicked on. The inner frame represents whether or
not the given player-character is selected; with different colours for
selected and unselected. The portrait is the image of the given
character.

Portrait images are separate from imagesets (an imageset is created for
each portrait at run-time in GUIManager::setPartyBarPortraits), which
should allow for user-selected images to be used as portraits in future
without extra scripting.

Not entirely pleased that the inner frame and portraits are implemented
by code (the new ImageWindow class) rather than by xml, but function's
my main priority.