This commit tackles the "context" buttons in Properties header, which were still using "ugly" old UI code.

It removes buttons_header.c file, adds a (small) space_properties.py one, with a PROPERTIES_HT_header class, which simply uses the RNA enum to draw the context buttons.
It also fixes that enum, btw, it always featured all contexts, which means you could (try to!) set through RNA invalid contexts...

Thanks to brecht and dingto for the reviews.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57584 954f8c5b-7b00-dc11-b283-0030488c597c