Make vtkSMProxyLocator customizable.

Making it easy for custom applications to customize state loading by
assigning proxies to ides for the proxy locator.

Change-Id: Idceb28f3836eb865f7d2f73855f7cb12c70205a3
Merge topic 'api_cleanup' into next

464467d6 Make vtkSMProxyLocator customizable.