Add support for lumino plugin deactivation

[fcollonval]

This add the support for the plugin deactivate method that will be support in JupyterLab 3.6.

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[bollwyvl]

y'all got any other breaking changes hidden up yer sleeve?

[bollwyvl]

(to be fair, many of these are very cool, but i'm really wondering about why these are being backported: every one of them reduces the chance that existing third-party extensions will work properly, which is going to lead to a lot of unhappy users on launch day)

[fcollonval]

@bollwyvl we don't use the deactivate in lumino application nor in JupyterLab. And if we start to do so (for instance for the extension manager), we should really first have a discussion about which core plugin will support it.

But for this project to work with a code like the one below, we need a proper way to deactivate the previous version. Otherwise reloading the plugin will fail due to the command ID being already registered.

import {
  JupyterFrontEnd,
  JupyterFrontEndPlugin,
} from '@jupyterlab/application';
import {
  DisposableSet
} from '@lumino/disposable';

const resources = new DisposableSet();

const plugin: JupyterFrontEndPlugin<void> = {
  id: 'mydynamicplugin',
  autoStart: true,
  requires: ['@jupyterlab/apputils:ICommandPalette'],
  activate: (app, palette) => {
    console.log('Hello from a dynamically loaded plugin!');

    // We can use `.app` here to do something with the JupyterLab
    // app instance, such as adding a new command
    let commandID = 'MySuperCoolDynamicCommand';
    let toggle = true;
    const cmd = app.commands.addCommand(commandID, {
      label: 'My Super Cool  Command',
      isToggled: function () {
        return toggle;
      },
      execute: function () {
        console.log('Executed ' + commandID + 'uu');
        toggle = !toggle;
      }
    });
    resources.add(cmd);

    const item = palette.addItem({
      command: commandID,
      // make it appear right at the top!
      category: 'AAA',
      args: {}
    });
    resources.add(item);
  },
  deactivate: () => {
    resources.dispose();
    resources.clear();
  }
};

export default plugin;

And incidentally this raises a good question, is it really easier to create a plugin for JupyterLab as it requires a deactivate function that is otherwise not needed. And that function will require good knowledge to be written properly.

[bollwyvl]

Again, it's important that these things get improved... but on X, rather than X.y. In a playground setting, exposing these things with docs is great and helpful.