Change OpenButton nomenclature to NavigationButton.

Change OpenButton nomenclature to NavigationButton.

Using NavigtionButton makes things more generic. Classes, methods and
variables have been changed.

This is a rename-only refactor.

* src/main/java/org/glom/web/client/activity/DetailsActivity.java:
* src/main/java/org/glom/web/client/ui/ListViewImpl.java:
* src/main/java/org/glom/web/client/ui/cell/NavigationButtonCell.java:
Renamed from OpenButtonCell.
* src/main/java/org/glom/web/client/ui/details/RelatedListTable.java:
* src/main/java/org/glom/web/client/ui/list/ListTable.java:
* src/main/java/org/glom/web/client/ui/list/ListViewTable.java: