Media Package, XML Registry, Programs, Cleanup

Added: Media related classes now have their own files in their own
package. No more Media.File!
Fixed: The JAXB annotations were cleaned up.
Added: A registry for removing the @XmlSeeAlso-annotations (they are
infelxible).
Added: Programs no longer are parts of the os, they now have to be
stored in files. They also hold an executor class (which will become
important when we introduce processes).
Added: General cleanup and small fixes, as usual.