Added appdata file

The appdata currently fails validation since the fedora validator
does not yet support multiple licenses for the metadata.

Fixes #596.
I would replace both paragraphs with what I wrote for the Qt Showroom page:

"Tiled is a general purpose 2D level editor with powerful tile map editing features. It’s built to be easy to use and is suitable for many type of games.

The Tiled map format is supported by a large amount of game engines and libraries. In addition, Tiled supports plugins to read and write map formats other than its own map format."