QtShellChannelMixin should emit kernel_info_reply signal not call it

QtShellChannelMixin incorrectly uses a callback approach to pass the
kernel_info_reply message, e.g.

   self._handle_kernel_info_reply(msg)

Qt kernels should emit Qt signals for this behaviour (as done elsewhere
in the class). This is fixed as follows, e.g.

   self.kernel_info_reply.emit(msg)

Relevant bug report: https://github.com/ipython/ipython/issues/6578