Merge pull request #258 from haraldpdl/product_admin_tabs

Product admin tabs
fix compatibility with last admin order class update
Merge pull request #287 from haraldpdl/admin_order_class_fix

fix compatibility with last admin order class update
fix compatibility with last admin order class update 2
Merge pull request #288 from haraldpdl/admin_order_class_fix2

fix compatibility with last admin order class update 2
Initial work to convert PayPal modules into a new PayPal App
Add hook to add PayPal tab to Admin->Orders page
Keep first GetExpressCheckoutDetails call in session to remove two repeated calls later in the checkout process
set $response_array global again (to be available in after_process())
remove order total check as the customer is forwarded to the checkout confirmation page anyway
Only pass shipping and taxes if Instant Update is enabled, otherwise only pass the subtotal
Skip the payment page after continuing from EC
Automatically select the first shipping rate available, not the cheapest.
onboarding development
Add "charset" parameter
Remove utf8_encode()'ing of parameter values
Remove charset as the stores value may conflict with what PayPal is expecting (utf-8 vs UTF-8; we can't strtoupper() the value as Shift_JIS is not all uppercase)
optimize code: no need to over POST parameters twice
optimize code: no need to over POST parameters twice
retrieve api credentials from our website when the store owner is returned to their admin
don't keep track of the generated merchant id, it must be regenerated each onboarding request
Merge Direct Payment (Payflow Edition) module into Direct Payment module
Add general Gateway configuration parameter to control processing of transactions through PayPal or Payflow
Add Payflow credentials configuration page to Manage Credentials page