The Cycles light linking branch is using the tree view UI but it seemed
to use the "wrong" layout. It wasn't clear that the layout has to be
reactivated before building the view.
Make it harder to use the API wrong now by requiring the layout as
argument, so the building can ensure it's active.
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).
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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
