Add a HostControl to enable hosting native controls

[cwensley]

This control would allow one to host native controls without having to create platform-specific assemblies.

This is mainly necessary for platforms such as WPF, where by default it has only a single HWND for the entire window.

[rafntor]

What is returned as NativeHandle for controls when targeting Wpf today; do all controls return the (same) Hwnd of the Main-window?

[rafntor]

Also, would this control also provide a 'setable' handle so that when interfacing to a native control that offers a handle for Eto to display can be supported (as opposed to Eto offering a handle for the native control to use) ?