Blender Git Loki

Add widget rollover feedback by implementing a Sub-Button pipeline

As pretty much agreed on in T38070, this implements widget-rollover for
better visual feedback.
The added Sub-Button pipeline should provide us a solid base for future
ideas like using Sub-Buttons to trigger operators (thinking of 'X'-icons
within tabs or menus e.g.), open popups/menus, provide better cursor
feedback, etc. They are basically independent buttons within buttons.

https://developer.blender.org/F187781