Added AppData description for Mypaint package

See here for more information: http://people.freedesktop.org/~hughsient/appdata/

This may need a few revisions down the line, but it's an okay starting
point.
Changed AppData image

The old one was a little inappropriate.
Merge branch 'appdata-xml'
Submodule bump.

Contains a big fix for OS X users: mypaint/libmypaint#98ed27