[RFC] BaseContainer now has a FileItemList

BaseContainer now has a FileItemList instead of a vector<GUIlistItemPtr>.
Reason behind this is, that you want to let skinner add lists everywhere (e.g. a Favourites list in library view). In order to do that we need to move content specific logic out of the CGUIWindows*.

Right now my Drag&Drop stuff uses this, to determine if a FileItemList is dropable and reorderable.
I also started to work on a context menu rfc where i wanted to move the decision of the visible items out of CGUIWindow*. There I need this one too.