UI: Animation editor scrubbing area

The main reason for this change is to allow setting the
active frame with the left mouse button, while still being
able to select e.g. keyframes with the same mouse button.

The solution is to introduce a new scrubbing region with
a specialized keymap. There are a couple of related todos,
that will be handled in separate commits.
Those are listed in D4654.

This solves T63193.

Differential Revision: https://developer.blender.org/D4654

Reviewers: brecht, billreynish

source/blender/editors/include/UI_resources.h

@@ -105,6 +105,7 @@ typedef enum ThemeColorID {
   TH_FACE_DOT,
   TH_FACEDOT_SIZE,
   TH_CFRAME,
+  TH_SCRUBBING_BACKGROUND,
   TH_TIME_KEYFRAME,
   TH_TIME_GP_KEYFRAME,
   TH_NURB_ULINE,
@@ -267,6 +268,8 @@ typedef enum ThemeColorID {
   TH_ICON_MODIFIER,
   TH_ICON_SHADING,
 
+  TH_SCROLL_TEXT,
+
   TH_NLA_TWEAK,       /* 'tweaking' track in NLA */
   TH_NLA_TWEAK_DUPLI, /* error/warning flag for other strips referencing dupli strip */