Expose widget visibility state from pqProxyWidgetDialog.

Make it possible for client code to check if any widgets are being shown
in the dialog.

Change-Id: I7a3b830a21a7b50ff8e3a35ec82f504c92c9ba2f
Merge topic 'refactor_python_tracing_2' into next

7fe1155c Expose widget visibility state from pqProxyWidgetDialog.