No user-visible changes expected.
Essentially, this makes it possible to use C++ types like `std::function`
inside `uiBut`. This has plenty of benefits, for example this should help
significantly reducing unsafe `void *` use (since a `std::function` can hold
arbitrary data while preserving types).
----
I wanted to use a non-trivially-constructible C++ type (`std::function`) inside
`uiBut`. But this would mean we can't use `MEM_cnew()` like allocation anymore.
Rather than writing worse code, allow non-trivial construction for `uiBut`.
Member-initializing all members is annoying since there are so many, but rather
safe than sorry. As we use more C++ types (e.g. convert callbacks to use
`std::function`), this should become less since they initialize properly on
default construction.
Also use proper C++ inheritance for `uiBut` subtypes, the old way to allocate
based on size isn't working anymore.
Differential Revision: https://developer.blender.org/D17164
Reviewed by: Hans Goudey
