Add API to send to primary window

[jcubic]

This can be implemented like this:

function sendToPrimary(data) {
    return sysend.list().then(list => {
        return list.find(window => window.primary);
    }).then(primary => {
        sysend.post(primary.id, data);
    });
}

It can be handy to allow:

sysend.post('primary', data);