Tracking: Highlight keyframes in path visualization

This gives better idea of what's going on with your track. Example:

{F693806}

Color of keyframes are configurable from theme editor of clip editor.

Reviewers: keir, brecht, Severin

Differential Revision: https://developer.blender.org/D2772

source/blender/editors/include/UI_resources.h

@@ -234,6 +234,8 @@ typedef enum ThemeColorID {
   TH_DIS_MARKER,
   TH_PATH_BEFORE,
   TH_PATH_AFTER,
+  TH_PATH_KEYFRAME_BEFORE,
+  TH_PATH_KEYFRAME_AFTER,
   TH_CAMERA_PATH,
   TH_LOCK_MARKER,