Introduce app.timer

[falkoschindler]

This PR implements one of our most-wanted feature requests #3225, an API for timing UI-independent function calls.

The current draft can be tested like this:

import time
from nicegui import app, ui

timer = app.timer(0.5, lambda: print(time.time()))
ui.button('Start', on_click=timer.start)
ui.button('Stop', on_click=timer.stop)
ui.switch('Running').bind_value(timer, 'running')

ui.run()

Open tasks:

• mypy on Python 3.8
• add parameter once: bool = False
• Should we start with sleeping or with calling the handler? ui.timer calls immediately, but RoSys' repeater doesn't: https://github.com/zauberzeug/rosys/blob/96da831c272f56daa664382e9ecb6dac159a4b85/rosys/rosys.py#L152 --> introduce parameter immediate: bool = True
• Do we need the complicated exception handling? --> No
• Do we need to call stop_all during shutdown?
• Use same terminology (enable, disable, active).
• pytest
• documentation

[rodja]

Looks good. I just modified the app.timer test a little bit. It was too similar to the ui.timer test on an auto-index page.