When this feature enabled:
* Frequency sets to 14MHz on esxdos activity - for fastest loading from
sd card;
* Frequency sets to 14MHz on 48 BASIC startup;
* Frequency sets to 3.5MHz within ~1ms after #FE port access - this
allows to load from tape within turbo mode. Also this improves beeper
sound effects;
* Otherwise user configured frequency is used.
* add 4.4 MHz and 5.2 MHz turbo modes
* handle magic key press if initialization wasn't completed before
* replace 'timings', 'ram', 'plus3' settings with one 'machine' setting
* refactor memory controller
* significantly improve classic timings
* magic rom: fix 'h' font character
* fix hanging of esxdos browser after magic key double press
* enable divmmc (esxdos OS) by magic rom on poweron; add NO-OS option
* improve pause ('f12' on ps/2 keyboard or 'start' on gamepad)
* magic rom: handle C-key on sega gamepad as exit
