* separate (ibm8514|xga)_enabled into (ibm8514|xga)_standalone_enabled
and (ibm8514|xga)_active, the former being enabled only for standalone
8514/A or XGA cards, the latter for all 8514/A and XGA-capable cards
and not saved into the config file;
* remove (ibm8514|xga)_has_vga and replace all uses of it with
(ibm8514|xga)_standalone_enabled;
* Qt UI: the checkboxes for standalone 8514/A and XGA are now correctly
grayed out if an (S)VGA card with 8514/A or XGA capability is selected,
including cases when the card is an internal/onboard one; said cards are
now no longer appear as SVGA multi-monitor compatible.
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