Adds an easy way to add drop support for tree-view rows.
Most of the work is handled by the tree-view UI code. The tree items can
simply override a few functions (`can_drop()`, `on_drop()`,
`drop_tooltip()`) to implement their custom drop behavior.
While dragging over a tree-view item that can be dropped into/onto, the
item can show a custom and dynamic tooltip explaining what's gonna
happen on drop.
This isn't used yet, but will soon be for asset catalogs.
See documentation here:
https://wiki.blender.org/wiki/Source/Interface/Views#Further_Customizations
This follows three main targets:
* Make creation of new tree UIs easy.
* Groundwork to generalize tree UIs (so e.g. Outliner, animation
channels, asset catalogs and spreadsheet data-sets don't have to
re-implement basic tree UI code) or even other data-view UIs.
* Better separate data and UI state. E.g. with this, tree-item selection
or the open/collapsed state can be stored on the UI level, rather than
in data. (Asset Catalogs need this, storing UI state info in them is
not an option.)
In addition, the design should be well testable and could even be
exposed to Python.
Note that things will likely change in master still. E.g. the actually
resulting UI isn't very nice visually yet.
The design is documented here:
https://wiki.blender.org/wiki/Source/Interface/Views
Differential Revision: https://developer.blender.org/D12573
