Blender Git Commit Log

UI: add optional tip callback to uiBut, and use it for per-item tooltips in UIList.

When defined, uiBut->tip_func is called when button's tip is generated. This allows
for advanced, dynamic generation of tooltips.

For now, only used by UIList, which can now optionaly use a given string property
of each item for its tooltip.

Thanks to Campbell for the reviews!