External Console: Add default value to 'open_python_at_startup' option

- This was preventing to open our Preferences pane after a first start.
Fix starting an external console at startup (which was broken since revision 827c5891e989)
IPython Console: Simplify getting the loading image path
Start an External console at startup before trying to start an IPython one

This was causing two erros:
1. Focus was not given to the IPython Console.
2. We were not associating the IPython client to its corresponding kernel widget.