Allow dragging and dropping query file into query sequence textarea.

All of the code is courtesy Clayton Wheeler (@csw) and Ben Woodcroft (@wwood),
(see #98) with very minor modifications from myself:
1. No need to dynamically position dnd-target-marker.
2. We already had a button to clear query sequences. Integrate with that.

Because of the recent frontend overhaul (including BS3 upgrade) it was easiest
for me to cherry-pick code, rather than merge csw:dnd or wwood:dnd branches.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>
Change drag n drop behavior.

* Drop anywhere.
* Show drop error on dnd overlay.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>