Does the following changes visible to users:
* Use panels and sub-panels to group elements.
* Use flow layout (single column by default).
* New layout uses horizontal margin if there's enough space.
* General reordering and renaming of buttons/panels/etc & icon changes.
* Move keymap related options from "Input" into own section.
* Own, left-bottom aligned region for //Save Preferences// button.
* Decrease width of Preferences window to suit new layout.
* Move buttons from header into the main region (except editor switch).
* Hide Preferences header when opened in temporary window.
* Don't use slider but regular number widget for UI scale.
* Use full area width for header.
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.
A detailed list of changed RNA paths should be (and rearranged UI
items?) done for the release notes.
Patch by @billreynish and myself.