Fix infinite feedback loop between Action states

[sharplet]

Fixes #220.

There is a potentially circular relationship between Action.isEnabled and Action.isExecuting. It's valid for users to observe properites on Action and feed the output back into the action state, and in turn influence Action.isEnabled.

This resolves an issue where:

1. The user supplies a condition as the action enabled state
2. Any property derived from the internal Action.state is observed
3. Synchronously, the observer updates the user-supplied state property
4. Which updates the internal state property
5. Which causes an externally-observable event to fire, returning to (3), indefinitely.

The reason for this manifesting as an infinite loop, rather than an infinite recursion, is the internal use of a replayLazily in Property: each time a side effect on the user-supplied property occurs, that queues up another event inside the replayed signal producer, and the observer is never attached because there is always a buffered event to consume.

The issue was that multiple events were firing synchronously when observing Action.isExecuting, even though the executing state had never changed. The fix is to skip repeats on Action.isEnabled and Action.isExecuting, so that observers are only ever notified if the value actually changes, instead of whenever something triggeres a side-effect on the internal state.

[mdiep]

Thanks for fixing this!