dnd: Skip drag window when drag starts

dnd: Skip drag window when drag starts

Summary:
Currently, when drag starts, drag window is not skipped. It makes ecore_x cannot find dropable window in some cases.

Issue: Dropable window is not found if we drop inside the initialized area of drag window.
(Example: in Dnd Types of elementary test, try to start dragging at right edge of left label and drop to left edge of right label. The drop is failed).
Reason: Ecore_x considers drag window when it tries to find dropable window at specific position. Therefore, it cannot find dropable window.
Resolve: Skip drag window.
@fix

Reviewers: raster, JackDanielZ, seoz

CC: woohyun, seoz
Differential Revision: https://phab.enlightenment.org/D710