When dropping file to sequencer timeline, coordinates for strip position
and overlap handling are used even if not set.
Reset internal state in on_drag_start callback and set is_modal
variable only if coordinates are updated. This way when dragging file
from external file browser, strip is added at current frame as before
modal operator was implemented.
Reviewed By: Richard Antalik
Differential Revision: http://developer.blender.org/D15333
3551b0a672