Blender Git Loki

UI: Initial User-Preferences redesign

Implements the first changes for T54115:

* Rename "User Preferences" window to "Settings" in the UI.
We'll likely put workspace settings in there, separate from the global
user settings. System settings should become separate from user
settings in future to allow settings for specific hardware.
* Add sidebar region for navigation (scrolls independently).
Addresses space problems, so we can add more categories as needed now.
* Increase size of Settings window to compensate new navigation bar.
* Group sections into User Preferences and System.
Icons for section groups by Andrzej Ambroz. Thanks!
* Bumps subversion for file compatibility.

Screenshot: https://developer.blender.org/F5715337

I also added categories for future work, but commented them out.
We may also want to redesign contents of each section now.

Reviewers: brecht, campbellbarton
Differential Revision: https://developer.blender.org/D3088
Design Task: https://developer.blender.org/T54115