Make Linux desktop experience better by adding and polishing desktop files and adding AppData

I've updated pronterface.desktop file a bit (added some categories as well). Also added desktop files for Plater and Pronsole.

While doing that I've also created AppData files - more info of them can be obtained from http://people.freedesktop.org/~hughsient/appdata/

Note that AppData requires some screenshot, so I've added them to the repository as well. Note to self: Recreate screenshots while connected to the printer.

The AppData files are licensed as CC0, as this is what AppData standard recommends. If you are against it, feel free to change the license filed to more strict content license, but keep it free as in freedom (choose eg. CC BY or CC BY-SA, not NC or ND).