UI: Theme: Shared regions

Continuing recent attempts to combine theme settings
and reduce complexity. part of #140360
This PR adds new root panel in theme called "Regions" which for now
includes two subpanels:

# Asset Shelf
This is just moving asset shelf from root to regions, where
it's more appropriate and less noisy.
No functional changes. (alternative to #144267)

# Channels
Previously known as "Theme Space List", or "Source List",
but in user interface we refer this region as "Channels".
This region is shared by all animation/video editors, and each had
a separate panel for theme properties. Now they're combined, and all
editors follow settings defined in Regions -> Channels.

Additionally, "Source List Title" property, which was never used is removed.
Properties are also renamed to be more intuitive and shorter.

# Toolbar / Sidebar
Previously was registered separately on every single editor. This allowed
for some extra customization, but in 5.0 need for this is reduced very much,
because elements drawn in those regions (tabs & tools) are also generalized,
so it makes sense to want same background color for them as well.

> This replaces 46 properties with 5, reducing the total by 41.
> Videos in PR

Pull Request: https://projects.blender.org/blender/blender/pulls/145609

source/blender/blenkernel/BKE_blender_version.h

@@ -27,7 +27,7 @@
 
 /* Blender file format version. */
 #define BLENDER_FILE_VERSION BLENDER_VERSION
-#define BLENDER_FILE_SUBVERSION 77
+#define BLENDER_FILE_SUBVERSION 78
 
 /* Minimum Blender version that supports reading file written with the current
  * version. Older Blender versions will test this and cancel loading the file, showing a warning to