Blender Git Loki

UI: Improve node group input / output list interface

This commit replaces the two-column list for editing node group inputs
and outputs with a cleaner solution with two panels. The new layout
has several benefits:
- It uses the vertical space in the node editor sidebar better.
- It should be more familiar because of similarity with other UI lists.
- It should look better with consistent alignment and icons.

Note that displaying the "Name" property outside of the list itself is
a bit inconsistent and could possibly be removed in the future.

Differential Revision: https://developer.blender.org/D9683