mcs48: add reset method, make A12 optional, unassorted adjustment

mcs48: add reset method, make A12 optional, unassorted adjustment

Add the reset() method which recently has become essential. Make the A12
"memory bank" address line optional. Use more Python idioms. Update
comments.

The control signals had to move, to avoid gaps between D7 and A8 as well
as between A11 and A12 in the GUI decoder properties dialog. With
dynamic assignment in the UI and with named references in the CLI this
shall not harm. Unmodified automated tests still pass.

The logic is prepared to handle data, address, and "bank" pin groups at
arbitrary locations, A[11:8] and D[7:0] need not be adjacent any longer.
Support for more than one memory bank pin is prepared, but the number of
bank pins needs to get determined at the start of decode(), when this
feature is to get added in the future.