Add lskeys helper script

#!/usr/bin/env perl

use strict;

use warnings;

use 5.010;

use autodie;

my @keys;

open(my $fh, '<', 'src/keyevents.c');

while (my $line = <$fh>) {

	chomp($line);

	if ($line =~ qr{

			if \s \( \! strcmp \( action , \s " (?<key> [^"]+ ) " \)\) }x) {

		push(@keys, $+{key});

	}

}

close($fh);

say join("\n", sort @keys);