Refine markers position operator

This operator will run a tracker from previous
keyframe to current frame for all selected markers.
Current markers positions are considering initial
position guess which could be updated by a tracker
for better match.

Useful in cases when feature disappears from the
frame and then appears again. Usage in this case
is the following:

- When feature point re-appeared on frame, manully
  place marker on it.
- Use Refine Markers operation (which is in Track
  panel) to allow tracker to find a better match.

Depending on direction of tracking use either
Forwards or Backwards refining. It's easy: if
trackign happens forwards, use Refine Frowards,
otherwise use Refine Backwards :)

release/scripts/startup/bl_ui/space_clip.py

@@ -292,6 +292,14 @@ class CLIP_PT_tools_tracking(CLIP_PT_tracking_panel, Panel):
         props.backwards = False
         props.sequence = False
 
+        col = layout.column()
+        col.label(text="Refine:")
+        row = col.row(align=True)
+        props = row.operator("clip.refine_markers", text="Backwards")
+        props.backwards = True
+        props = row.operator("clip.refine_markers", text="Forwards")
+        props.backwards = False
+
         col = layout.column(align=True)
         props = col.operator("clip.clear_track_path", text="Clear After")
         props.action = 'REMAINED'