Add desktop and AppData files

This commit adds a desktop and AppData file to Heimdall-frontend, this way the application can show up in the applications menu and software centers in Linux.

The desktop file should be placed in /usr/share/applications/heimdall.desktop and the AppData in /usr/share/appdata/heimdall.appdata.xml

Feel free to rewrite the text or change the snapshots as you want.

For more info, see: http://people.freedesktop.org/~hughsient/appdata/