8514/A and XGA handling refactor:

* 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.

src/include/86box/86box.h

@@ -130,8 +130,8 @@ extern int    isamem_type[];          /* (C) enable ISA mem cards */
 extern int    isartc_type;            /* (C) enable ISA RTC card */
 extern int sound_is_float;        /* (C) sound uses FP values */
 extern int    voodoo_enabled;         /* (C) video option */
-extern int    ibm8514_enabled;        /* (C) video option */
-extern int    xga_enabled;            /* (C) video option */
+extern int    ibm8514_standalone_enabled; /* (C) video option */
+extern int    xga_standalone_enabled; /* (C) video option */
 extern uint32_t mem_size;         /* (C) memory size (Installed on system board) */
 extern uint32_t isa_mem_size;     /* (C) memory size (ISA Memory Cards) */
 extern int      cpu;              /* (C) cpu type */

src/include/86box/vid_xga_device.h

@@ -17,7 +17,6 @@
 
 #ifndef VIDEO_XGA_DEVICE_H
 #define VIDEO_XGA_DEVICE_H
-extern int xga_has_vga;
 
 #ifdef EMU_DEVICE_H
 extern const device_t xga_device;

src/include/86box/video.h

@@ -208,7 +208,8 @@ extern double cpuclock;
 extern int    emu_fps;
 extern int    frames;
 extern int    readflash;
-extern int    ibm8514_has_vga;
+extern int    ibm8514_active;
+extern int    xga_active;
 
 /* Function handler pointers. */
 extern void (*video_recalctimings)(void);