Add "One-step Checkout" functionality (Modern Store theme only)

1. create ord:OnGetPaymentGatewayAjax event, that will return: - 1 h a set of hidden fields to be placed on form (tag ord:GetFormHiddenFields does that) form submit url (tag ord:GetFormAction does that) 2. on payment gateway change (e.g. PayPal button is clicked): - 0.5 h remove all hidden fields with "payment-gw-field" class from the form add hidden fields (if any returned) retrieved from ord:OnGetPaymentGatewayAjax event (also set "payment-gw-field" HTML class to added hidden fields) change form action url to the one, returned by ord:OnGetPaymentGatewayAjax event 3. when shipping address fields are changed then (they all are filled-in) retrieve dropdown (call to new "checkout/shipping_type" template) for given order - 0.5 h 4. when "same as billing" is checked and user is entering billing address, then instantly copy that address to shipping address fields (that will be hidden at the moment) and trigger shipping address change code (#3) - 0.2 h 5. when when "same as billing" is checked/unchecked then trigger shipping address change code (#3) - 0.2 h 6. if no shipping types can be found based on entered address then create "fake shipping type dropdown" with "Please enter address" option selected - 0.5 h 7. update "shipping cost" and "order total" totals (without ajax query) when shipping cost is changed - 0.1 h 8. when no shipping types available then set "shipping cost" to 0 and update "order total" accordingly - 0.5 h 3.5 h