Add an enter_* callbacks for widget toolkit consumers

[samdroid-apps]

These callback is useful for widget toolkit users of the render, which need to insert a widget at when entering (as well as exiting) some of the block or table types. This is what we are using in something-for-reddit which uses the Gtk+ widget toolkit. For example, we add a table in the "enter_table" callback, and mark the table as completed during the "table" callback.