Fix saveViewState() for folderviews with CheckListItems, like TAlbumListView and TagFilterView.

With current implementation, we are not able to have one common loadViewState() and saveViewState() method, so we need to re-implement them in TAlbumListView and TagFilterView. This is not nice because we have duplicate code now, but since we are going to switch to ModelView concept next release, it would be a waste of time to create a new common interface or abstract class now.

We can not call virtual methods in constructors / destructors, so every view inherited from FolderView needs to call saveViewState() in its own destructor instead.