qtutils: store whether files exist when creating tree widget items

Signed-off-by: David Aguilar <davvid@gmail.com>
doc: add v2.0.1 release notes draft

Signed-off-by: David Aguilar <davvid@gmail.com>
status: do not show some actions when files do not exist

Hide the "Launch Editor" and similar actions when the selected files do
not exist.

Closes #238

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