LPS-27530 Bookmarks portlet changes for new staging

Hey Julio,

Here comes the bookmarks changes for the new staging.

Few notes for the review:
- the NAMESPACE attribute in the BookmarksPortletDataHandler has public scope now as listed in the commit message we are thinking about a bigger refactoring for the different constants across the staging
- The PortletDataContext class itself and how we are using it will face a refactor as well. Namely we will move it to the base class soon as we identify all the necessary requirements/purposes for it.

Thanks,

Máté

cc: @KocsisDaniel