Move appdata.xml

The target directory /usr/share/appdata seems to be deprecated now.

More details in this bug and here

Debian 9 supports /usr/share/metainfo.

Ubuntu 18.04 supports /usr/share/metainfo. Ubuntu 16.04 and 14.04 seems only to use /usr/share/appdata but Gnome is able to find the Datovka entry. Don't know whether it recognises files in /usr/share/metainfo or whether it does some other magic.

Centos 7 and recent Fedoras support /usr/share/metainfo.

Info about Debian needs to be researched.