UI: Make uiBut safe for non-trivial construction

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

source/blender/editors/interface/interface_context_menu.cc

@@ -934,7 +934,7 @@ bool ui_popup_context_menu_for_button(bContext *C, uiBut *but, const wmEvent *ev
     uiButViewItem *view_item_but = (uiButViewItem *)ui_view_item_find_mouse_over(region,
                                                                                  event->xy);
     if (view_item_but) {
-      BLI_assert(view_item_but->but.type == UI_BTYPE_VIEW_ITEM);
+      BLI_assert(view_item_but->type == UI_BTYPE_VIEW_ITEM);
       UI_view_item_context_menu_build(C, view_item_but->view_item, uiLayoutColumn(layout, false));
       uiItemS(layout);
     }