Add support for IPython Jupyter QT Console as a debugging aid

If manuskript is launched with its last argument set to "--console" an
interactive console opens up to help debug the application.
The IPython, qtconsole and matplotlib libraries must be installed for it
to work and they won't get imported unless the argument is passed to the app.

manuskript/main.py

@@ -91,14 +91,58 @@ def prepare(tests=False):
 
     return app, MW
 
-def launch(MW = None):
+def launch(app, MW = None):
     if MW is None:
         from manuskript.functions import mainWindow
         MW = mainWindow()
 
     MW.show()
 
-    qApp.exec_()
+    # Support for IPython Jupyter QT Console as a debugging aid.
+    # Last argument must be --console to enable it
+    # Code reference : 
+    # https://github.com/ipython/ipykernel/blob/master/examples/embedding/ipkernel_qtapp.py
+    # https://github.com/ipython/ipykernel/blob/master/examples/embedding/internal_ipkernel.py
+    if len(sys.argv) > 1 and sys.argv[-1] == "--console":
+        try:
+            from IPython.lib.kernel import connect_qtconsole
+            from ipykernel.kernelapp import IPKernelApp
+            # Only to ensure matplotlib QT mainloop integration is available
+            import matplotlib
+
+            # Create IPython kernel within our application
+            kernel = IPKernelApp.instance()
+            
+            # Initialize it and use matplotlib for main event loop integration with QT
+            kernel.initialize(['python', '--matplotlib=qt'])
+
+            # Create the console in a new process and connect
+            console = connect_qtconsole(kernel.abs_connection_file, profile=kernel.profile)
+
+            # Export MW and app variable to the console's namespace
+            kernel.shell.user_ns['MW'] = MW
+            kernel.shell.user_ns['app'] = app
+            kernel.shell.user_ns['kernel'] = kernel
+            kernel.shell.user_ns['console'] = console
+
+            # When we close manuskript, make sure we close the console process and stop the
+            # IPython kernel's mainloop, otherwise the app will never finish.
+            def console_cleanup():
+                app.quit()
+                console.kill()
+                kernel.io_loop.stop()
+            app.lastWindowClosed.connect(console_cleanup)
+
+            # Very important, IPython-specific step: this gets GUI event loop
+            # integration going, and it replaces calling app.exec_()
+            kernel.start()
+        except Exception as e:
+            print("Console mode requested but error initializing IPython : %s" % str(e))
+            print("To make use of the Interactive IPython QT Console, make sure you install : ")
+            print("$ pip3 install ipython qtconsole matplotlib")
+            qApp.exec_()
+    else:
+        qApp.exec_()
     qApp.deleteLater()
 
 def run():
@@ -111,7 +155,7 @@ def run():
     app, MW = prepare()
     # Separating launch to avoid segfault, so it seem.
     # Cf. http://stackoverflow.com/questions/12433491/is-this-pyqt-4-python-bug-or-wrongly-behaving-code
-    launch(MW)
+    launch(app, MW)
 
 if __name__ == "__main__":
     run()