gs-app: Split ‘unknown’ state out of app size values into separate type

Rather than representing an ‘unknown’ size as 0 and an ‘unknowable’ size as G_MAXUINT64, this changes all the size properties in GsApp to represent them as (GS_SIZE_TYPE_UNKNOWN, _) and (GS_SIZE_TYPE_UNKNOWABLE, _), and to represent known/valid sizes as (GS_SIZE_TYPE_VALID, size).

This allows 0 byte sizes to be represented consistently, which is important for indicating that an app doesn’t need any additional downloads (if GsApp:size-download is 0, for example).

Signed-off-by: Philip Withnall pwithnall@endlessos.org