base: refactor signal_transmitter::submit

Move it to platform specific .cc file, so that it may get re-implemented
platform specifically if needed.

Issue #1446

repos/base/include/base/signal.h

@@ -34,6 +34,7 @@ namespace Genode {
 	class Signal_transmitter;
 	class Signal;
 	class Signal_dispatcher_base;
+	class Signal_connection;
 	template <typename> class Signal_dispatcher;
 }
 
@@ -122,6 +123,8 @@ class Genode::Signal_transmitter
 
 		Signal_context_capability _context;  /* destination */
 
+		Signal_connection * connection();
+
 	public:
 
 		/**