drag and drop #98: highlighted drag box now the correct size

Signed-off-by: Ben J. Woodcroft <donttrustben near gmail.com>
small UI fixes 2 and 3 for drag and drop as suggested by @yannickwurm in #98

Signed-off-by: Ben J. Woodcroft <donttrustben near gmail.com>
warn that drag and drop replaces the existing query, as suggested by @yannickwurm in #98

Signed-off-by: Ben J. Woodcroft <donttrustben near gmail.com>
drag and drop: when dropped file is rejected, don't leave the query sequence box readonly

Signed-off-by: Ben J. Woodcroft <donttrustben near gmail.com>
Fix drag and drop box positioning for chrome

Signed-off-by: Ben J. Woodcroft <donttrustben near gmail.com>