Currently, only Lightgroups that exist in the current view layer can be
selected from object or world properties.
The internal UI code already has support for search fields that accept
unknown input, so I just added that to the API and use it for lightgroups.
When a lightgroup is entered that does not exist in the current view layer
(e.g. because it's completely new, because the view layer was switched or
because it was deleted earlier), a new button next to it becomes active and
adds it to the view layer when pressed.
Differential Revision: https://developer.blender.org/D14540
Moves all `interface_region*` files to C++ except for the tooptip region
which is slightly more complicated. Also move a few other files as well.
This helps to simplify and speed up code, especially through the use
of better C++ data structures. This change builds on all platforms on
the buildbot.
