define translatable shared constants for UNKNOWN and other fields

[Victor Martinez]

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...
}