Allow to open attachments by double-click

Summary:
I didn't feel like stubbing away that one for testing since it jumps
straight at QTemporaryFile and QDesktopServices here. Probably better to
keep it as a simple ten lines method in that case.

Reviewers: dfaure, bensi, franckarrecot

Reviewed By: dfaure

Differential Revision: https://phabricator.kde.org/D7265