New panels ideas and porting existing

[jettify]

I've looked around for cool panel in pyramid/django/flask projects:

1. git history: https://github.com/sjhewitt/django-debug-toolbar-git-panel
2. plot tables relations for sqlalchemy: https://github.com/uralbash/pyramid_debugtoolbar_sadisplay
3. replay ajax calls: https://github.com/jvanasco/pyramid_debugtoolbar_ajax

May be we should add some library like: aiohttp_debugtoolbar_extras or own repository for each task.

[RealJTG]

Missing templates panel for aiohttp_jinja2, obviously.

[RealJTG]

Currently trying to port Jinja2 debug panel.
I've added on_jinja2_template_rendered signal to aiohttp_jinja2:

def setup(app, *args, app_key=APP_KEY, context_processors=(), **kwargs):
    ...
    app.on_jinja2_template_rendered = signals.Signal(app)
    ...

def render_string(template_name, request, context, *, app_key=APP_KEY):
    ...
    asyncio.ensure_future(app.on_jinja2_template_rendered.send(app, context))
    ...

and subscribed on it in new debug panel:

class Jinja2DebugPanel(DebugPanel):
    """
    A panel to display Jinja2 template variables.
    """
    name = 'Jinja2'
    has_content = True
    template = 'jinja2.jinja2'
    title = 'Jinja2'
    nav_title = title

    def __init__(self, request):
        super().__init__(request)
        # attach handler
        request.app.on_jinja2_template_rendered.append(self.on_template_rendered)

    def on_template_rendered(self, app, context):
        self.populate(context)
        # detach handler
        app.on_jinja2_template_rendered.remove(self.on_template_rendered)

    def populate(self, context):
        self.data = {'variables': []}
        for name, value in context.items():
            self.data['variables'].append((name, repr(value)))

Works pretty similar to flask-debugtoolbar except they don't need to detach handler each time because there's only one request.
I don't like this approach - weird things happens if two or more handlers left attached (e.g. due to exception between panel initialization and template rendering), but can't figure out how/where to put single handler and how to return captured context. Any suggestions?

[asvetlov]

Calling async signals from sync render_string looks weird.