[Spec] Add IDispatcher to the IAnimatable interface

[hartez]

Description

Add IDispatcher to IAnimatable interface.

The animation methods in Forms automatically handle UI thread marshaling for the user. Unfortunately, they do this using the static Device.IsInvokeRequired and Device.BeginInvokeOnMainThread, which are incompatible with multi-window applications on some platforms. To address this, we can add a dispatcher to the IAnimatable interface to use for these thread checks.

API

On Xamarin.Forms.IAnimatable, add IDispatcher Dispatcher { get; }

Backward Compatibility

Anyone who has implemented the IAnimatable interface will need to add an IDispatcher to their implementation.

Difficulty : Low

The main implemnentation of IAnimatable in Forms is VisualElement, which already has an IDispatcher. The other implementations (Tizen's NavigationDrawer and a couple of test classes) will need to add it as well. The explicit type check in the DoAction method of AnimationExtensions will need to be removed, and the use of the static Device methods will need to be replaced with Dispatcher.