Anim: VSE: Add overlay to visualize scene strip range when using scene syncing

This adds an overlay to the animation editors to visualize the current scene strip
in and out points in the scene timeline when the `Sync Scene Time` option in
the sequencer is used.

* Adds an overlay toggle + panel to the dopesheet.
* Adds a new theme setting (for the overlay) under
  `Common` > `Animation` > `Scene Strip Range`.
* Adds the overlay drawing to the dopesheet.

Pull Request: https://projects.blender.org/blender/blender/pulls/146165

release/datafiles/userdef/userdef_default_theme.c

@@ -312,6 +312,7 @@ const bTheme U_theme_default = {
     .anim = {
       .playhead = RGBA(0x4772b3ff),
       .preview_range = RGBA(0xa14d0066),
+      .scene_strip_range = RGBA(0x00000080),
       .channels = RGBA(0x194e8080),
       .channels_sub = RGBA(0x0f2c4d80),
       .channel_group = RGBA(0x1a332d37),