pqMultiViewWidget: use a new QLayout subclass instead of hierarchy of QSplitters
pqMultiViewWidget creates a hierarchy of QSplitters to support multi view layout. This results in a lot of reparenting of widgets when the layout changes. Change of parent is not the lightest thing to do for OpenGL-based widgets. Hence it makes sense to revisit the implementation and use a new QLayout subclass instead that or a new QSplitter like QWidget that handles the layout without nesting all the widgets under child widgets.