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
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