non-python kernels run python code with qtconsole

When launching the qtconsole with a non-python kernel it crashes immediately since the frontend [requests that the following code is run by the kernel](https://github.com/ipython/ipython/blob/master/IPython/qt/console/ipython_widget.py#L276):

    try:
        _usage
    except:
        from IPython.core import usage as _usage
        get_ipython().register_magic_function(_usage.page_guiref, 'line', 'guiref')
        del _usage

obviously only python kernels understand and can execute this.

Either we
- check the language of the kernel first (with a `kernel_info_request` message).
- find another way to register the magic functions for python kernels.