Moved show command to normal.c.

We don't generate the show command for non bound mappings. This might save cpu
cycles. The show commands for the normal mode are set from the keypress
handler. So we can handle show commands like vim where an incomplete mapped
command like ':nmap foo 12g', where the '12g' should appear in show command as
incomplete command.

Now the show commands keys are displayed like in vim with <hex> for none
printable keys.