status: improve drag and drop behavior

status: improve drag and drop behavior

Support shells that do not gracefully handle URLs by including only a
plain text payload when "shift" is held down during a drag operation.

Improve the default behavior of our QMimeData payloads. We were using
some workarounds to support older versions of gnome-terminal that
required utf-16 encoding of mimedata URLs.

Modern versions do not need these workarounds. We can use utf-8 across
the board and everything works as expected.

Remove the cola.dragencoding configuration variable since it is no
longer needed.

Use only QMimeData::setText() and QMimeData::setUrls() instead of
manually building the QMimeData payloads using QMimeData::setData().

Closes #719
Reported-by: Taylor Braun-Jones <taylor@braun-jones.org>
Signed-off-by: David Aguilar <davvid@gmail.com>