WIP: UI Tweaks for the timeline
This MR includes several tweaks for the timeline.
- Adds a darker background for the timeline. This helps separate the parts better.
- Makes the dividing lines (spacer) more subtle, so they don't stand out so much
- Change the colors for Audio clips to the new HIG colors (Green 1 and Green 3).
- Also invert the contrast in audio clips so they look better with a dark background
- Add a CSS class for Title Clips and a specific color for them (orange 1 from the HIG)
- Change the keyframe and line color to the new HIG colors (yellow 1)
- Increase the dark overlay for keyframes and make the line a bit more subtle
- Use symbolic icon for zoom
