qtutils: make the drag and drop encoding configurable

While gnome-terminal expects utf-16 encoded paths, other terminals, e.g.
terminator, expect utf-8.  There's no way we can sensibly handle this
situation without making the encoding configurable.

Add a cola.dragencoding variable that can be used to control the
encoding used during the drag and drop operation.  The default value is
utf-16, but suggest utf-8 in the documentation.  Mention terminator
since it is known to have a problem with the utf-16 default value and
works correctly when set to utf-8.

utf-8 would have been a better default if gnome-terminal were not the
default terminal in most Linux distributions.

Related-to: #373
Signed-off-by: David Aguilar <davvid@gmail.com>