Previously UI view items would support custom drop controllers (so they
could react to data being dragged over them and dropped). This is now
more generalized so the views themselves can do this as well.
Main changes:
- Support calculating a bounding box for the view, so this can be used
for recognizing mouse hovering.
- Rename "drop controller" to "drop target", this is more clear, less
abstract naming.
- Generalize drop controllers/targets. There is a new
`ui::DropTargetInterface` now.
- Add support for drop targets in the `ui::AbstractView` base class, so
custom views can use this.
Pull Request: blender/blender#105963
