This patch adds a left aligned sidebar to the spreadsheet editor. This
Sidebar can be used to navigate the geometry component types and
attribute domains. It also provides a quick overview of domain sizes.
It replaces the two dropdowns in the regions header.
Next step will be to add the domain cycling shortcut
using the CTRL + mouse wheel.
Reviewer: Dalai Felinto (dfelinto), Julian Eisel (Severin),
Hans Goudey (HooglyBoogly).
Differential Revision: https://developer.blender.org/D11046
