UI: Make selected track color in Movie Clip Editor themeable

Selected tracks in the Movie Clip Editor's Dope Sheet used hard-coded color.
Added a new theme property for them, and previously hard-coded color is now just a default.

Pull Request: https://projects.blender.org/blender/blender/pulls/144265

scripts/presets/interface_theme/Blender_Light.xml

@@ -1033,7 +1033,7 @@
         strips_selected="#ff8c00"
         metadatabg="#000000"
         metadatatext="#ffffff"
-        preview_range="#a14d0066"
+        track_selected="#e7f7e74d"
         >
         <space>
           <ThemeSpaceGeneric