New event loop integration system

I would like to change our event-loop integration with Trio's [guest mode](https://trio.readthedocs.io/en/stable/reference-lowlevel.html#using-guest-mode-to-run-trio-on-top-of-other-event-loops). Trio's guest mode also works on asyncio thanks to [aioguest](https://github.com/oremanj/aioguest/tree/master/aioguest). For it to work, we just need to have a function that can schedule a callback in the host event-loop in a thread-safe way.
- in asyncio, that would be `loop.call_soon_threadsafe(callback)`,
- in tkinter, that would be `tk.after(0, callback)`.
- Qt supports Trio's guest mode, see [here](https://doc.qt.io/qtforpython-6/examples/example_async_minimal.html).
- ...

For instance, here is how it looks like when the host event-loop is tkinter's:
```py
from tkinter import Tk

from anyio import sleep
from anyioutils import start_guest_run

def run_sync_soon_threadsafe(fn):
tk.after(0, fn)

def done_callback(outcome):
print("done")

async def foo():
for i in range(10):
await sleep(0.1)
print(i)

tk = Tk()
start_guest_run(foo, run_sync_soon_threadsafe=run_sync_soon_threadsafe, done_callback=done_callback)
tk.mainloop()
```
Before calling `tk.mainloop()`, we call `start_guest_run()` which runs `foo` as a task in tkinter's event-loop.

Thoughts?