Added slow-mem aliasing on ECS for the 512k+512k config.
Internally no more than 512k chipmem will be used on OCS.
Moved blitter_ECS setting to new file chipset.cpp and renamed option. It is only in the code, the config format is not changed.
Made common functions to mask chipset pointers and chipmem access. Old constants removed and made configurable.
UI: Require reset for OCS/ECS change.
UI: For now, validating the chip memory setting on load config and start.