Correct version patching of Preferences theme

In a77b63c56943eb, the Preferences navigation region background was
made brighter. Recently stored userpref.blends (since b00963afc14978b,
so beta release included) would still use the slightly darker
background for the Preferences navigation region.
Now the version patch added for a77b63c56943eb also sets the new color
for those recent configs.

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

Use full region width for file path panels

The horizontal margin would shrink the text button, reducing space to
show file paths in. For file paths, larger buttons make sense though, so
remove the margin.

Cleanup: PEP 8

Merge branch 'master' into userpref_redesign

If anim player is not custom, gray out path option

Merge branch 'master' into userpref_redesign

Avoid empty panel bl_label

And remove unused operator execution context change.

Merge branch 'master' into userpref_redesign

Fix invisible scrollbars in Preferences

Need to force correct updates of action-zone alphas for dynamically
sized regions.