Blender Git Loki

UI: Draw real node sockets for node input buttons

For buttons representing node inputs (e.g. in the material properties)
rather than drawing some generic socket icon, the actual sockets are
drawn now. That includes color, shape and the selection outline.

This should make it easier to understand what these buttons relate to.

Screenshots: {F8469252}, {F8469248} (The left alignment will be done in
a follow-up commit.)

Differential Revision: https://developer.blender.org/D7409

Reviewed by: Brecht Van Lommel, Cl�ment Foucault, William Reynish