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/blenloader/intern/versioning_userdef.c

@@ -151,6 +151,8 @@ static void do_versions_theme(const UserDef *userdef, bTheme *btheme)
   {
     FROM_DEFAULT_V4_UCHAR(space_file.execution_buts);
     FROM_DEFAULT_V4_UCHAR(tui.icon_folder);
+    FROM_DEFAULT_V4_UCHAR(space_clip.path_keyframe_before);
+    FROM_DEFAULT_V4_UCHAR(space_clip.path_keyframe_after);
   }
 
 #undef FROM_DEFAULT_V4_UCHAR