ui/app: drop Event

input.Event is enough if we stretch "input" to mean both input
devices and other events such as profiling events and system
commands.

The pointer and key packages are separate already, so I don't
expanding the meaning is unreasonable.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/pointer/pointer.go

@@ -182,5 +182,4 @@ func (s Source) String() string {
 	}
 }
 
-func (Event) ImplementsEvent()      {}
-func (Event) ImplementsInputEvent() {}
+func (Event) ImplementsEvent() {}