a800.xml, a800_flop.xml: initial pass at QA

[angelosa]

• bus/a800: add preliminary Corina ROM cart support;
• bus/a800: bbsb bankswitch works on reads too, makes it playable;
• atari/atari400.cpp: bulk replace cart tags into m_cartleft / m_cartright, move config_ntsc_screen / config_pal_screen to parent state class;
• atari/maxaflex.cpp, atari/bartop52.cpp: convert to screen raw params;
• atari/atari400.cpp: fix a130xe extended RAM out of bounds accesses, fixes t34 after disk swap;

[angelosa]

1. Need to update a800.xml for stuff that got fixed along the way;
2. Need to finish a800_flop.xml;
3. State machine and memory map modernization goes in another feature branch, it's pretty clear from adding Corina alone that this slot system won't cut it for the later way more complex cart configs.