Sequencer: 2D cursor for the preview & transform

- Use 2D cursor in the preview space using shortcuts
  matching the UV editor and 3D view.
- Add Cursor tool, cursor transform.
- Support for cursor and bound-box pivot.
- Add pivot pie menu.

source/blender/editors/include/ED_util.h

@@ -30,6 +30,7 @@
 extern "C" {
 #endif
 
+struct GPUBatch;
 struct Main;
 struct bContext;