Bug fix:  6845 was not considered when restoring from a saved snapshot.
Rename variables for more clarity, and altered a function signature to match other functions that handle updating state based on the port variables.
Use 6845 MA12 and MA13 signals with cassette operation.
We sample these from the Address Start register (R12), as they are never changed by the Lynx's screen.
Removal of _level variable now we know the official cassette enable features.
Bring 6845 port handling into line with other ports handling, re: optimising away processing on "no change".