Input::Binding for non-C++ language bindings

Quietly insert forward declaration of a Input::Binding class, and make
it a friend of Input::Event and Input::Session_client. This is to allow
non-C++ language bindings (Nim) to access private members by providing
their own implementation of the Binding class.

Fix #2889

repos/os/include/input/event.h

@@ -41,6 +41,7 @@ namespace Input {
 	struct Touch_release   { Touch_id id; };
 
 	class Event;
+	class Binding;
 }
 
 
@@ -74,6 +75,8 @@ class Input::Event
 		template <typename R, typename T>
 		static Genode::Point<R> _xy(T const &a) { return Genode::Point<R>(a.x, a.y); }
 
+		friend class Input::Binding;
+
 	public:
 
 		/**

repos/os/include/input_session/client.h

@@ -31,6 +31,8 @@ class Input::Session_client : public Genode::Rpc_client<Session>
 		Genode::size_t const _max_events =
 			_event_ds.size() / sizeof(Input::Event);
 
+		friend class Input::Binding;
+
 	public:
 
 		Session_client(Genode::Region_map &local_rm, Session_capability session)