Submit and validation for base_settings_form()

Add base_settings_form_submit() and base_settings_form_validate() functions for a checkout panes settings form. Here's an example of where this would be useful:

The Commerce Giftwrap module requires a price field on it's settings form page. As far as I can tell this should be done with the functions commerce_currency_amount_to_decimal() and commerce_currency_decimal_to_amount() to switch between the minor value to be stored in the DB and the decimal value for users. This requires some more handling than just setting the value in the variables table. In reality the submit function can be added manually through a form alter but I feel it would be cleaner if it was automatic.

There's more discussion on the issue here: #1448144: Commerce Giftwrap.