Draw the background of the asset shelf header fully transparent, with an opaque
background with rounded corners behind sections containing buttons. This
reduces the visual space consumed by the asset shelf, and makes the header
follow a tabbed folder metaphor better. Also, this works much better with our
click-through feature, where transparent parts of regions without buttons are
passed through the region under it (we might want to consider unifying code
here a bit).
The edge to drag for region resizing respects the transparent sections.
When there is little space between sections, the sections get merged so that
there are no small gaps in the bar.
Part of #107881.
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Note that the core of this is implemented in a generic way, so this can be
reused for other regions.
Pull Request: https://projects.blender.org/blender/blender/pulls/112241
73460903fa