define translatable shared constants for UNKNOWN and other fields

Some strings are used very extensively, and are often not translated. In many places, they are used for comparisons. Although comparing strings is usually a bad practice, we could get rid of this problem by defining all the commonly used values as constants. In this case, these would be defined in Media.vala. Personally, I would suggest dedicating a separate file for this purpose (namely common-defs.vala) to avoid having constants stored in unknown files.

@common-defs.vala:

namespace BeatBox {
    public static const string UNKNOWN_ALBUM = N_ ("Unknown Album");
    // and so on...
}