Define the commited API for Shapefile

We should define the first elements of what would be the committed API for Shapefile. The intend of this task is not to define the full API neither to change implementation details that are not visible through the API. The intend is only to reduce the size of the public API to elements that, we think, have good chances to stay there.

This task is targeted for completion before SIS 0.5 release. The intend is to give some guarantees to Apache SIS 0.5 users that their code is likely to compile and run with Apache SIS 0.6. To achieve this goal, the proposal is to hide the API elements which are likely to change, for preventing users to depend on it. Those API elements can be hidden either by declaring them package-privated, or by moving them in any org.apache.sis.internal sub-package, depending on which approach is the less disruptive for the code.