Create qBittorrent.appdata.xml

fixes #1701

An appdata file for inclusion in the upcoming software centers as per the new freedesktop.org specs.

It should be placed in /usr/share/appdata/ similar to the way .desktop files are placed in /usr/share/applications/, e.g. if you have a "$(datadir)/applications" definition in your makefiles, you need to add a "$(datadir)/appdata" as well.

Please, skim through the file in case I made a mistake and please, include it in the 3.1.x branch as well.
Of course you are free to modify it as you see fit, e.g. change the license, change copyright info, use your own screenshots, etc., just make sure it passes validation. 

Thanks!

http://people.freedesktop.org/~hughsient/appdata/
http://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html