Making pqContextView use the vtkSMSelectionHelper to create new selection proxy
instead of the custom code. The custom code was not working correctly when the
current selection source was not of the expected type. vtkSMSelectionHelper
handles all those cases.

Also removing obsolete code pqSelectionManager for creating selection sources.
Use vtkSMSelectionHelper instead.

Change-Id: Ia994af7cf04f2f9f97b202b10b6a617b96f2af83