patch 8.0.0451: some macros are in lower case

Problem:    Some macros are in lower case.
Solution:   Make a few more macros upper case. Avoid lower case macros use an
            argument twice.

src/gui_gtk.c

@@ -1522,7 +1522,7 @@ split_button_string(char_u *button_string, int *n_buttons)
 	    else if (*p == DLG_HOTKEY_CHAR)
 		*p++ = '_';
 	    else
-		mb_ptr_adv(p);
+		MB_PTR_ADV(p);
 	}
 	array[count] = NULL; /* currently not relied upon, but doesn't hurt */
     }