Make sidebar tabs use the `wcol_tab` widget colors, instead per editor
settings. Simplifying theme tweaking and the following benefits:
* Set colors in one place, affect all tabs.
* Support for text colors for inactive and active tabs.
* Support for the new `Outline Selected` property.
* In the future sidebar tabs could support `Shaded` style.
The tabs region background color remains per-editor, to be able to
customize it in a way that fits the surrounding colors (sidebar region,
header, or navigation bar background).
See pull request for screenshots and details.
Pull Request: https://projects.blender.org/blender/blender/pulls/140288
e8735c3203