VSE: Strip drawing improvements

This patch include changes:
- Thicker and clearer selection indication
- Slimmer handles
- More transparent muted strips
- Trim frame number is drawn inside the strip
- Strip text is drawn in upper part of strip
- Color strips now have specific color, with chosen color drawn under strip text
- Transition strip will use color of input strips showing direction of transition
- Selecting effect strip will highlight input strips
- Selecting multicam strips will highlight target channel
- Missing media state is now indicated by a red line drawn on the top part of the strip
- A checkerboard pattern is now drawn on the outsides of the meta range
- Hold still regions are now always drawn if existent, with a darker shade of the strip’s background color

Author: Alessio Monti di Sopra <a.monti>

Reviewed By: ISS

Differential Revision: https://developer.blender.org/D6883

release/scripts/presets/interface_theme/blender_light.xml

@@ -779,10 +779,13 @@
         image_strip="#8f744b"
         scene_strip="#828f50"
         audio_strip="#4c8f8f"
-        effect_strip="#598f4c"
-        transition_strip="#8f4571"
+        effect_strip="#4c456c"
         meta_strip="#5b4d91"
+        mask_strip="#8f5656"
         text_strip="#824c8f"
+        color_strip="#8f8f8f"
+        active_strip="#d9d9d9"
+        selected_strip="#ff6a00"
         frame_current="#5680c2"
         time_scrub_background="#292929e6"
         keyframe="#ff8500"