Blender Git Loki

Fix UI bug with 'socket menus' (used e.g. by cycles' Material panels).

Also tweaked a bit how nodes with more than one socket available are drawn:
* The node entry itself (a label) has no more any icon, so that it is aligned with the other single-socket nodes.
* The sub-entries (i.e. node's sockets) are more clearly indented (using a blank icon).

Details:
Another case of bad internal UI_HAS_ICON flag erasing by brutally assigning an uneeded value to but->flag.
Revealed by recent split of some button flags into but->drawflag.

Thanks to Thomas Dinge for noting the issue.