This currently shows panels that were in the 2.79 3D view toolbar
which are now popovers.
In some cases it's useful for these to stay open.
This commit adds a space type to do this.
Note this is currently empty in object mode.
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.
