Camera tracking: tracks copy/paste operator

This commit implements basic clipboard support for movie tracking data
int clip editor. Used own implementation of clipboard like it's done
for sequencer.
Ideally it needed to be switched to more general clipboard system, but
currently this system is designed for text data only and it need to
be re-designed itself. But this feature is quite useful since object
tracking is implemented, so it should be OK to live with such own
implementation for a while.

release/scripts/startup/bl_ui/space_clip.py

@@ -855,6 +855,10 @@ class CLIP_MT_track(Menu):
         layout.separator()
         layout.operator("clip.clean_tracks")
 
+        layout.separator()
+        layout.operator("clip.copy_tracks")
+        layout.operator("clip.paste_tracks")
+
         layout.separator()
         props = layout.operator("clip.track_markers",
             text="Track Frame Backwards")