widgets: add text/plain and text/uri-list mimedata hints

Specify that we provide text/plain and text/uri-list for better drag-
and-drop behavior with external apps.

Closes #373
Reported-by: Taylor Braun-Jones <taylor@braun-jones.org>
Signed-off-by: David Aguilar <davvid@gmail.com>
drag-and-drop: generate better mimedata for gnome-terminal

Generate a valid text/x-moz-url mimedata entry, encoded in
utf-16, so that gnome-terminal gets the right paths when
receiving the mimedata from git-cola.

This should improve drag-and-drop for most applications.

Closes #373
Reported-by: Taylor Braun-Jones <taylor@braun-jones.org>
Signed-off-by: David Aguilar <davvid@gmail.com>
doc: update v2.0.8 release notes draft

Signed-off-by: David Aguilar <davvid@gmail.com>
Merge branch 'drag-and-drop'

* drag-and-drop:
  doc: update v2.0.8 release notes draft
  drag-and-drop: generate better mimedata for gnome-terminal
  widgets: add text/plain and text/uri-list mimedata hints

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