status: teach the status widget to send files to the trash

Use the excellent cross-platform send2trash module to allow sending
untracked files to the trash.

Make "Ctrl+Backspace" send files to the trash, and
"Shift+Ctrl+Backspace" remove files with confirmation.

"Ctrl+Backspace" performs the traditional remove action when send2trash
is not available.

Closes #341

Suggested-by: Taylor Braun-Jones <taylor@braun-jones.org>
Thanks-to: Virgil Dupras <hsoft@hardcoded.net>
Signed-off-by: David Aguilar <davvid@gmail.com>