Enable writing JupyterLab plugins using Flexx.

At the moment, this needs bleeding edge versions of Flexx (and also Jupyterlab?).

• Download or clone the source from Github.
• pip install -e .

• flexxlab enable to set things up.
• flexxlab add flexx.ui.examples.mondriaan.Mondriaan is one example.
• flexxlab add my.module.MyModelSubclass to use your own Flexx widget.
• jupyter lab as usual.

• Use flexxlab enable to set things up. After updating Flexx, you'll want to run this again to install the latest flexx-core.js into Jupyterlab.
• Use flexxlab disable to turn it all off again.
• Use flexxlab add x.y.MyModel to register a Flexx Model/Widget as a plugin.
• Use flexxlab remove x.y.MyModel to turn individual plugins off.

Plugins are simply a subclass of flexx.app.Model or flexx.ui.Widget. It should preferably have a jlab_activate() method in JS, and can also implement JLAB_AUTOSTART and JLAB_REQUIRES. For example:

class MyPlugin(ui.Widget):
    
    def init(self):
        
        # Popluate the widget as usual
        with ui.HBox():
            self.labale1 = ui.Label(text='bla bla')
            self.button1 = ui.Label(text='Press me')
            ui.Widget(flex=1)  # spacer
    
    class JS:
        
        JLAB_AUTOSTART = True
        JLAB_REQUIRES = ['jupyter.services.file-browser',
                         'jupyter.services.document-registry']
        
        def jlab_activate(self, lab, fb, dr):
            self.title = 'Flexx foo demo'
            
            # Use fb (file browser) and dr (document registry) ...

See https://github.com/zoofIO/flexxlab/tree/master/flexxlab/examples for more examples. See the Flexx documentations to learn more about creating widgets with Flexx.