Nicegui

- introducing `on_page_ready` callback for ui.page
- bugfies
- made sure our own print statements are shown
- fixed rendered texture side
- added missing ui update for markdown
- cleaner DOM for `ui.notify`

- introduction of `scene.text` and `scene.text3d` to render text in 3d
- fixed timer update behaviour

- introducing disconnect-handlers for ui and page
- timer.interval is now a bindable property
- removed support for the obsolete keypress event
- improved documentation
- fixed firefox update issues and other bugfixes

From 0.8 onwards we changed the update strategy of NiceGUI:

Prior to this release, we used and propagated the same strategy as JustPy: event handlers needed to return `False` to not trigger full page updates. This works good for small examples. But if your UI gets bigger, full page updates become a real performance bottle neck. Adding `return False` everywhere is quite cumbersome and error prone. Instead, NiceGUI now smartly updates only the element which changes. And for cases where it can not simply detect a change the user can use `ui.update(...)` to enforce an update. See our [documentation for more details and examples](https://nicegui.io/#ui_updates).

- introducing `ui.add_head_html()` and `ui.add_body_html()`
- updated lifecycle example to include `ui.on_connect`