Folders Panel fixes

The following functionality from Dolphin 1.x has
been ported to the new view-engine:
- Allow expanding/collapsing of items
- Automatically select the current item
- Context menu for items

Related improvements to the view-engine:
- Make the expanding/collapsing interface already accessible
in the base classes KItemModelBase and KItemListView. If
no expanding/collapsing is supported at all by derived models
(which is usually the default case) simply not reimplementing
those 3 methods is sufficient and it does not introduce an
additional complexity like in QAbstractItemModel/QModelIndex.

- Automatically handle the expanding/collapsing in KItemListController.
This also includes the key-handling, which is quite special for
expandable items.

- Don't let KItemListView automatically scroll to the current item
if the current item got changed. The automatic scrolling should
only be done if the current item has been changed by the user.
Hence this functionality has been moved to the KItemListController
which currently only triggers the automatic scrolling if the current
item has been changed by the keyboard (we might extend the usecases
later if required).