PR to integrate Paymill

[henobi]

Hi,
this PR adds a basic integration of Paymill into the isotope shop system.
It is still work-in-progress as there are some details missing. Apart from that payments should work fine.

[henobi]

Here are all the required translation fields. I post it here because I don't have a transifex account.

//payment_paymill.php
$GLOBALS['TL_LANG']['payment_paymill']['translation_details'] = 'Paymill Transaktion: Zahlungsdetails';
$GLOBALS['TL_LANG']['payment_paymill']['order_id'] = 'Bestellungs-ID';
$GLOBALS['TL_LANG']['payment_paymill']['paymill_code'] = 'Paymill Code';
$GLOBALS['TL_LANG']['payment_paymill']['paymill_status'] = 'Paymill Status';
$GLOBALS['TL_LANG']['payment_paymill']['paymill_id'] = 'Paymill ID';


//tl_iso_payment.php
$GLOBALS['TL_LANG']['tl_iso_payment']['paymill_legend'] = 'Paymill-Einstellungen';
$GLOBALS['TL_LANG']['tl_iso_payment']['paymill_public_key'] = [
    'Öffentlicher Schlüssel',
    'Geben Sie hier den öffentlichen Schlüssel (public key) für Ihren Paymill Account ein.'
];
$GLOBALS['TL_LANG']['tl_iso_payment']['paymill_private_key'] = [
    'Privater Schlüssel',
    'Geben Sie hier den privaten Schlüssel (public key) für Ihren Paymill Account ein.'
];

[aschempp]

Unfortunately, we cannot require a Composer dependency in Isotope, because that dependency would not be available in the extension repository. Would it be possible to implement the API without the Paymill dependency? Otherwise this would need to be an extension :-(

[aschempp]

I'm closing this as it seems stalled. Feel free to reopen if we want to continue.