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

release/datafiles/userdef/userdef_default_theme.c

@@ -793,6 +793,7 @@ const bTheme U_theme_default = {
     .path_keyframe_after = RGBA(0xc4c4ffff),
     .gp_vertex_size = 1,
     .metadatatext = RGBA(0xffffffff),
+    .track_selected = RGBA(0xe7f7e74d),
   },
   .space_topbar = {
     .back = RGBA(0x18181800),