Made Tiled registering *.tmx as MIME-type

Tiled now registers the MIME-type application/x-tiled based on the file suffix
*.tmx. It also registers itself as application being able to handle this
MIME-type. This means systems following the freedesktop standard [1] will now
display tmx files with a tiled icon and open those files with tiled.

[1] http://standards.freedesktop.org/shared-mime-info-spec/shared-mime-info-spec-latest.html