Add KeePassX launcher to the panel

Originally created by @alant on #6290 (Redmine)

It would be nice to have a keepassx launcher in the top panel along with the webbrowser, pidgin, etc. One using it to store all its passwords is likely to need it during every working session. However, opening it every day from the Applications -> Accessories menu is quite painful. Furethermore, it’s one of the only applications which we want to promote everyday use with Tails that doesn’t already has its launcher and can’t be opened by double-clicking on a file icon.

Feature Branch: feature/keepassx_launcher