qt: Adds a global option in preferences to use usr_path as the default

open directory for image file selection.

src/86box.c

@@ -183,6 +183,7 @@ int confirm_save = 1;				/* (C) enable save confirmation */
 int	enable_discord = 0;			/* (C) enable Discord integration */
 int pit_mode = -1;				/* (C) force setting PIT mode */
 int fm_driver = 0;				/* (C) select FM sound driver */
+int open_dir_usr_path = 0;                      /* default file open dialog directory of usr_path */
 
 /* Statistics. */
 extern int mmuflush;