Introduce observable collections

New Features

• New observable collections. ObservableList[S] lets you collect stuff in a list which you can observe for changes. ValueList[S] lets you add Value[S], of which changes you will also be notified when observing the collection
• Also Added ObservableIntList, IntValueList, ObservableStrList and StrValueList, which provide useful helper methods
• Variable[S] not has set_delay_notify, a context manager which allows you to set a value, but not have its listeners notified immediately
• New classes ValuesObservable and ValuesEvent, which allows observing iterables, either as an Iterable or as each individual element
• Added emit_lazy method to all Events, allowing you to create values, only when the event really is listened to
• Observables now have various map methods, to map to other observers from other Observables with a mapping function. Events will know if all their observers are themselves unobserved Observables and not emit even then.
• Value[S]s can now additionally be observed with functions that take two parameters. In that case, the first is the new value and the second is the old value

Improvements

• Deriving Values from other Values will return a Constant if it is derived from only constants
• Constant are now equal to each other if their value is equal

Bug fixes

None

Breaking Changes

• Value[S]s are no longer Observable themselves. Instead, they now provide a observable property
• Renamed Value[S].is_dependent_on to Value[S].is_derived_from