Support creating background tasks from awaitables

[falkoschindler]

In zauberzeug/rosys#178 we noticed that background_tasks.create doesn't accept awaitables as promised by the type annotation:

class MyAwaitable:
    def __await__(self):
        yield
        print('Awaiting...')

my_awaitable = MyAwaitable()

ui.button('Run', on_click=lambda: background_tasks.create(my_awaitable))

This PR replaces the assertion asyncio.iscoroutine(coroutine) and applies asyncio.wait_for as a wrapper if needed.