Nicegui

- functions decorated with `ui.page` can optionally receive a request object
- new example to show working with the request object to implement authentication: [examples/authentication](https://github.com/zauberzeug/nicegui/tree/main/examples/authentication)

- improved styleability of joystick
- better naming of header/footer for customised pages
- add test for customised page
- provide access to justpy view within customised pages

- The `ui.page` decorator is now a class. This enables you to encapsulate common styling and behaviour for all pages. See the [customisation example](https://github.com/zauberzeug/nicegui/blob/0bfd587e26be57548bca95ead40298e44a00d1e3/examples/customisation/custom.py#L6-L20) for further details.
- Pytests are now executed as GitHub Action for every push.

- using private page generator for main.py
- fix style definitions with trailing semicolon 93
- test/demo for on_page_ready callback (see 50 and 94)

New Way to Define Pages

Pages are not created through a contextmanager anymore. Instead a decorator is used to configure a page builder. This is much closer to FastAPI and the existing get-decorator.

Example:

python
ui.page('/')
def index_page():
ui.label('Hello, world!')


It also allows us to create private pages: Before this change, it was quite complicated to not share all state with all browsers (eg. dialogs and menus). See https://nicegui.io/#page for documentation and 85 for more details on the decision process.

Other Changes

- introduced Selenium based pytests
- only update class, style and props if they have changed

The UI is now stoppable with `ui.shutdown` (see 83). Example:

python
from nicegui import ui

ui.label('Your UI...')
ui.button('Quit', on_click=ui.shutdown)
ui.run(reload=False)

print('Do something else...')