Support for UTF-8 strings in labels/title

[zmughal]

Due to using syswrite() strings must be passed as UTF-8 encoded bytes:

An example where this is a problem with a workaround:

#!/usr/bin/env perl

use utf8;
use Encode;
use PDL;
use PDL::Graphics::Gnuplot;
use PDL::Constants qw(PI);

sub plot_sin {
	my ($w, $coeff) = @_;
	my $xrange = [ -2*PI, 2*PI ];
	my $x = zeroes(1e3)->xlinvals(@$xrange); my $y = sin($coeff * 2 * PI * $x );

	my $title = "y = sin( $coeff * 2π  * x )";
	# to get around sending text through syswrite()
	my $title_octets = encode('UTF-8', $title);

	$w->plot(with=>'lines', $x, $y, {
		xrange => $xrange,
		title => $title_octets, # can not pass $title as-is
	});
}

[zmughal]

I came across this again even though I was the one that created the issue.

Perhaps when the string sent to syswrite() matches /[^\x00-\xFF]/, then we know we are not dealing with octets so a more informative error could be thrown.

Here's something that illustrates another less intrusive workaround:

use utf8;

use PDL;
use PDL::Constants 'PI';
use PDL::Graphics::Gnuplot;

my $gp = gpwin();
{ no utf8; # removing "no utf8" causes it to fail with "Wide character in syswrite"
    $gp->plot({'xlabel', "θ"}, zeros(100)->xlinvals(-PI, PI)->sin);
}
$gp->pause_until_close;

[mohawk2]

Look good! Could you PR it?