DispatcherTimer should implement the IDisposable interface

We've learned the hard way that if `Stop()` is not called on `DispatcherTimer` instances, once they are started, they continue to live forever, even if nothing references the instance anymore.

After looking at the implementation of `DispatcherTimer`, we noticed that when calling `Start()`, a reference to the `DispatcherTimer` is added to the `Dispatcher.CurrentDispatcher` (which is static) and the reference is only removed once the `Stop()` method is called.
Now knowing that `Stop()` must always be called in order to avoid memory leaks, shouldn't the class implement `IDisposable` and make sure to call `Stop()` in the `Dispose()`? This would indicate the consumer of the class that it needs to be disposed for resources to be released.

Same way `System.Threading.Timer` implements `IDisposable`, I think `DispatcherTimer` should as well.