Support for begin/update/end callbacks allowing state to be cached
and reused while dragging a number button or slider.
This is done using `UI_block_interaction_set` to set callbacks.
- Dragging multiple buttons at once is supported,
passing multiple unique events into the update function.
- Update is only called once even when multiple buttons are edited.
- The update callback can detect the difference between click & drag
actions so situations to support skipping cache creation and
freeing for situations where it's not beneficial.
Reviewed by: Severin, HooglyBoogly
Ref D11861
8839b4c32a