This patch adds the drag and drop strip previews in the VSE.
It also adds two new functions to the drag and drop API.
1. "draw_in_view" for callbacks that wants to draw elements in local viewport coordinates
2. "on_drag_start" that can be used for prefetching data only once at the start of the drag.
Reviewed By: Julian, Campbell
Differential Revision: http://developer.blender.org/D14560
77794b1a7b