make BasicScreen::{Enter,Space}Pressed() pure virtual

src/clock.h

@@ -42,6 +42,9 @@ class Clock : public Screen<Window>
 		virtual void Update();
 		virtual void Scroll() { }
 		
+		virtual void EnterPressed() { }
+		virtual void SpacePressed() { }
+		
 		virtual bool allowsSelection() { return false; }
 		
 		virtual List *GetList() { return 0; }