Adapt deliteful to recent changes in delite (leverage attachedCallback() instead of startup(), removal of underscore prefix for protected members)

This PR is to start collecting adaptations after delite's recent changes:
* Removal of underscore prefix for protected properties (https://github.com/ibm-js/delite/commit/672db9f9ecfb90870fd7914cab2f7f8f835db49f )
* "widgets should leverage attachedCallback() and not startup()" (https://github.com/ibm-js/delite/commit/01a0439d472a5a9c2f2fb69b6a307b0f2a0cf358)

Remarks:
* This PR is NOT to be merged now (because deliteful still depends on the old delite release; we first need a new delite release).
* With the current PR, all deliteful unit tests pass except PageableList's. This still needs to be investigated and fixed.