add left, right arrows for tree navigation in preferences side bar, xml dialog, etc (treeview lists)

Summary:

Right/left arrow keys dont work for tree navigation (expand/collapse) in preferences side bar

Steps to reproduce:

• open Inkscape
• Edit > Preferences (ctrl + shift + p)
• try to navigate the side bar with arrow keys on keyboard

What happened?

Up and down keys work, but right/left keys for expanding/collapsing the tree structure dont.

What should have happened?

Right key should expand an entry, left key should collapse one.

Version Info:

- Inkscape Version: 1.1-dev (60f6cd6, 2020-09-21) / all?
- Operating System: Win 10 1903

Search term: expand/collapse/expand