fixes #3354 - Position not saved on logbook screen

The root cause of this issue is that the scroll positions of the ListViews showing the logbook and the friends logbook are not persistet when flipping more then two pages away - i.e. when the ViewPagerAdapter destroys the corresponding views and reinstantiates them later.

Therefore, I added a methods to the PageViewCreator interface to save and restore the state of the encapsulated view, provided an empty implementation in the AbstractCachingPageViewCreator to be compatible with all existing page views, and a concrete implementation for the CacheLogsViewCreator. Saving and restoring the view states is done in the ViewPagerAdapter.