Move all non view rendering logic from frontend plugins to wrapper models

Many CMS frontend plugins contain code which is not directly related to rendering/controlling the view and so should be in model classes instead. Examples: interpretation of JCR node types, finding an hippo:document's associated hippo:handle, etc.

A number of NodeModelWrapper subclasses already exist for these purposes, these should be further extended to contain this logic. They also should be made "type safe", i.e. they can only wrap nodes of certain types so this should be checked inside their constructors.

See discussion on mailing list [1].

[1] http://www.nabble.com/-9825--hippo-ecm-trunk-addon-cms-prototype-frontend-src-main-java-org-hippoecm-cmsprototype-frontend-plugins-list%3A-CMS7-331%3A-Messaging-between-Tasklist-and-BrowsePerspective-plugins-%28tree%2C-documentlisting%2C-workflow%2C-variants%29-to14732880.html