LibGUI: Add a MultiView widget, based on FileManager's "DirectoryView"

A MultiView is a combination of ItemView, TableView and ColumnsView
smashed into a single widget. You can switch between the view modes
by calling MultiView::set_view_mode().

Note that MultiView inherits from StackWidget, not AbstractView.
That's purely for practical reasons, although I'm not entirely sure
if there would be some benefit to having it inherit from AbstractView.

Libraries/LibGUI/Forward.h

@@ -53,6 +53,7 @@ class Model;
 class ModelEditingDelegate;
 class ModelIndex;
 class MouseEvent;
+class MultiView;
 class PaintEvent;
 class Painter;
 class ResizeCorner;