Add various GUI pieces (exit button, resource counts, unit details).
Clean up implementation of button panels.
Split some content out of 'common' and into page-specific files.
Delete some obsolete content.
This was SVN commit r7337.
CGUI now represents a single 'page'. CGUIManager maintains multiple
pages and switches between them.
Split the XML files into pregame, loading, session and messagebox pages.
Added hotloading of GUI pages.
Minor GUI cleanups.
(Merge from hg 81862d33780c)
This was SVN commit r7214.
