Blender Git Loki

UI: Preferences Redesign Part 2

(Part 1 was 00963afc14978b)

Does the following changes visible to users:
* Use panels and sub-panels for more structured & logical grouping
* Re-organized options more logically than before (see images in D4148)
* Use flow layout (single column by default).
* New layout uses horizontal margin if there's enough space.
* Change size of Preferences window to suit new layout.
* Move keymap related options from "Input" into own section.
* Own, left-bottom aligned region for Save Preferences button.
* Adjustments of names, tooltips & icons.
* Move buttons from header into the main region (except editor switch).
* Hide Preferences header when opened in temporary window.
* Use full area width for header.
* Don't use slider but regular number widget for UI scale.
* Gray out animation player path option if player isn't "Custom"

Internal changes:
* Rearrange RNA properties to match changed UI structure.
* Introduces new "EXECUTE" region type, see reasoning in D3982.
* Changes to panel layout and AZone code for dynamic panel region.
* Bumps subversion and does versioning for new regions.

RNA changes are documented in the release notes:
https://wiki.blender.org/wiki/Reference/Release_Notes/2.80/Python_API/Preferences_API

Design & implementation mostly done by @billreynish and myself.
I recommend checking out the screenshots posted by William:
https://developer.blender.org/D4148#93787

Reviewed By: brecht

Maniphest Tasks: T54115

Differential Revision: https://developer.blender.org/D4148