Generate nonnull default wrappers for list getters

Generate nonnull default wrappers for list getters

This adds the knowledge about default methods to BindingMapping,
templates and CodeHelpers, so that whenever we emit a List accessor
(getFoo) we also emit a corresponding nonnull accessor (nonnullFoo).

The nonnull accessor is a default method, which takes invokes
getFoo() and runs it through CodeHelper.nonnull(), which turns
nulls into empty lists.

JIRA: MDSAL-18
Change-Id: Ic4fc80aa071b7d5b93bf49d1de44855f4bd820f2
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>