Manage subscriptions for dynamic properties

This implements subscription to and unsubscription from properties that are created and deleted at runtime for quiddities.

This is done first by creating a get_quiddity_by_id function which manages a cache of Pyquiddity instances. This function should always be called instead of calling sw.get_quid because the later causes a new Pyquiddity instance to be created and callback subscriptions are registers for every Pyquiddity and a switcher Quiddity can be associated for multiple Pyquiddity instances.

All the calls to sw.get_quid are transformed to get_quiddity_by_id to make sure we are not creating new Pyquiddity instance. Note that this mitigates #212 (closed) because it makes it very unlikely that switcherio instanciates more Pyquidditys than necessary.

This adds reset_property_subscriptions that removes all property subscriptions for a quiddity and immediately resubscribes to all available properties.

This function is called when an info tree value is grafted or pruned at an adress of the form property.<something> which ensures that we have up to date callbacks no matter the order in which we receive the events from the backend. (Its a bit brute force but it works)

Some unused buggy "pyquid specific signal" code was also removed from pyquiddity.