Blender Git Loki

Proper implementation of "Make Asset" for button context menus

Buttons or other UI elements can now set a "focused_id" context pointer. The
asset operators (and in future, general data-block operators) can use this
then. Further the UI code automatically sets it when a button represents a
data-block pointer. That way, data-block search menus or data-block selectors
also show "Make Asset" in the context menu.

This also works for material slots now, the material slot you right click will
properly set the "focused_id" context pointer, and "Make Asset" makes that
material an asset.

Had to add support for using custom context pointers to context menus and
UILists.

Addresses T82664.