Added the following new machines:

- Olivetti M19.
- Olivetti M240.

Implemented mainboard jumper settings for Olivetti M24 machine:
- Like on a real M24, onboard video card can be disabled, and other video cards can be installed.
- Renamed m_olivetti_m24.c to m_xt_olivetti.c, as it handles all XT-class Olivetti machines.

Decoupled Olivetti OGC (640x400 extended CGA) from M24 machine code:
- OGC exists both as 16-bit proprietary card and 8-bit ISA card (GO708).
- Implemented underlined text handling in 80x25 character mode.
- Like Compaq CGA, replaced code portions common to CGA with calls to vid_cga.c.

src/include/86box/video.h

@@ -241,6 +241,10 @@ extern const device_t gd5480_pci_device;
 extern const device_t compaq_cga_device;
 extern const device_t compaq_cga_2_device;
 
+/* Olivetti OGC */
+extern const device_t ogc_device;
+extern const device_t ogc_m24_device;
+
 /* Tseng ET4000AX */
 extern const device_t et4000_isa_device;
 extern const device_t et4000k_isa_device;