Updated minetest.appdata.xml to follow new spec

The AppData spec has been updated to ask for a few new things, including project license, on top of the already asked for AppData license. The Licenses now follow the SPDX spec, found here: http://spdx.org/licenses/ and the spec update can be seen here: http://blogs.gnome.org/hughsie/2014/05/02/appdata-meet-spdx-spdx-meet-appdata/

I'll outline here, for clarification, which licenses I have included,
and for what reason, in alphabetical order.

Apache-2 - In Minetest we package DroidSansFallback, which is under the Apache 2 License.

CC-BY-SA 3.0 - The license for Minetest assets including sound, and textures.

LGPL 2.1+ - The license for Minetest code

MIT - The license for Jthread, which we package with minetest.