add Appdata file and update desktop file #178
This adds an appdata file for qtpass. The appdata file generally gets used by software centers to display things such as the description and screenshots.
This also adds the GenericName key to the desktop file and the language keys for both GenericName as well as Comment.
The appdata file gets used by gnome-software and other software centers, the specification can be found here: https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html