Make python shell dockable.

1. Makes pqPythonShell a widget that is fairly self-contained. Now one can create as may pqPythonShell instances as needed.
2. ParaView simply places a pqPythonShell in a dock widget to add Python shell to the application.
3. pqPythonShell does not init Python immediately, but does so when the shell gets focus or attempts to execute some Python snippet.
4. When load Python state, or run macros, they will now run in the global interpreter context, rather that in the Python shell.
5. Cleanly handles capturing of output/error messages or input request coming from code executing in Python shell or else where e.g. macros, or loading of Python state.