Overhaul plat_get_string()

Now takes constants with human-readable names instead of
Win32 string table IDs, language.h is no longer needed

ui_msgbox*() no longer accepts string IDs as arguments directly,
plat_get_string() must be explicitly called to retrieve the string

src/network/net_plip.c

@@ -26,7 +26,6 @@
 #include <stdarg.h>
 #define HAVE_STDARG_H
 #include <86box/86box.h>
-#include <86box/language.h>
 #include <86box/lpt.h>
 #include <86box/timer.h>
 #include <86box/pit.h>