1) Renamed vtkSMPart to vtkSMOutpuPort.
2) Replaced vtkSMCompoundProxy with vtkSMCompoundSourceProxy which makes it
possible to create compound proxies with multiple outputs/inputs.
3) Moved all the complex logic for managing filters/sources menus from
pqMainWindowCore to pqProxyMenuManager.
4) As far a proxy manager is concerned, there's no difference between compound
proxies and regular proxies. Now PXM flaunts API to register custom proxies
which can be a compound proxy or a regular proxy. Both are created using the
same NewProxy API.