Checkout pane configuration should be self contained

Since http://drupalcode.org/project/ubercart.git/commit/5a4c637 checkout pane plugins are instantiated with any necessary configuration settings instead of having to read the full checkout config. Where possible we should move settings from CheckoutSettingsForm so they are self contained inside each plugin.

It might also be useful to allow checkout panes to depend on each other, for example shipping quotes depends on delivery address - then we could move "Hide delivery information when carts have no shippable items" to the delivery address pane settings, and it would automatically disable the shipping quotes pane as well.