Continues and depends on #145609, part of #140360 project.
Previously, all 5 animation/video editors had different properties
for Scrubbing/Markers region. This PR adds new Region panel for it,
so that all editors can share one color.
Text color in this region was coming from regular editor text color.
To differentiate between editors (as is the pattern already with other editors),
and allow more modular themeability of regions, new Text color is added
for that region as well.
Video in PR
Pull Request: https://projects.blender.org/blender/blender/pulls/146113
250384dd57
