Improve input sensor selection in config flow

[rautesamtr]

The current sensor selection isn't ideal yet especially with a lot of valid entity entries. See #121

Find a better solution.

[jnxxx]

It seems like a great integration, but how can I use an attribute as input?

Like this: state_attr('weather.mylocation', 'humidity')

[rautesamtr]

So revisiting this since we now have entity selectors available for config_flows. Those would resolve #141 with copy pastable id's and offer us text search. So the question remains how much we should still filter the available entities in the list or not.

[rautesamtr]

@jnxxx You can always create template sensors with the attribute as state and use that as input.