Move UI Styles regions settings into a dedicated page

Problem/Motivation

When a design system has a lot of styles and CSS variables (if UI Skins is present too), the PHP max_input_vars is easily reached.

Proposed resolution

Split UI Styles settings into a dedicated page (it will still be save in the theme setting).

Provide menu links: Appearance / Regions styles:
- theme X
- theme Y
- theme Z
- etc.
Provide menu local tasks.

Only if the theme has styles.

User interface changes

Settings splitted, new menu links, new local tasks.