This changes `UI_but_func_tooltip_set` so that it allows passing a custom free function, which has two benefits:
* The caller can pass `null` to indicate that the value should not be freed.
* Arbitrary c++ data can be passed to the callback (before the struct had to be trivially destructible).
I added `uiFreeArgFunc` and used it in other places where appropriate.
Differential Revision: https://developer.blender.org/D11738
17a67bf778