Add SDL mapper support for F13-F24 (as in, future support for generating
IBM PC F13-F24 scan codes). The reason the keyboard emulation does
nothing so far, is that there are several different descriptions of the
scan codes out there, one saying that F13-F24 are transmitted as
SHIFT+F1-F12, another that F11-F20 are SHIFT+F1-10 and F21-F22 are
CTRL+F1-F2. Until I can work out which one to emulate, the f13-f24
bindings do nothing at the keyboard level, though they are stubbed out
to prevent DOSBox from exiting with an error about unknown scan codes.