This patch removes a bunch of specific code for drawing the spreadsheet
data set region, which was an overly specific solution for a generic UI.
Nowadays, the UI tree view API used for asset browser catalogs is a much
better way to implement this behavior.
To make this possible, the tree view API is extended in a few ways.
Collapsibility can now be turned off, and whether an item should
be active is moved to a separate virtual function.
The only visual change is that the items are now drawn in a box,
just like the asset catalog.
Differential Revision: https://developer.blender.org/D13198
01df48a983