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/demo/src/app/scout/browser_window.cc

@@ -136,7 +136,7 @@ class Iconbar_event_handler : public Scout::Event_handler
 		/**
 		 * Event handler interface
 		 */
-		void handle(Event &ev)
+		void handle_event(Event const &ev) override
 		{
 			static int key_cnt;