Adapt high-level components to new parent API

This patch adjusts the various users of the 'Child' API to the changes
on the account of the new non-blocking parent interface. It also removes
the use of the no-longer-available 'Connection::KEEP_OPEN' feature.

With the adjustment, we took the opportunity to redesign several
components to fit the non-blocking execution model much better, in
particular the demo applications.

Issue #2120

repos/os/include/framebuffer_session/framebuffer_session.h

@@ -23,6 +23,7 @@ namespace Framebuffer {
 
 	struct Mode;
 	struct Session;
+	struct Session_client;
 }
 
 
@@ -81,6 +82,8 @@ struct Framebuffer::Session : Genode::Session
 {
 	static const char *service_name() { return "Framebuffer"; }
 
+	typedef Session_client Client;
+
 	virtual ~Session() { }
 
 	/**