LibCore: Fix broken "stay_within" mechanism in event dispatch

The "stay_within" parameter to CObject::dispatch_event() optionally
specifies a node in the CObject parent chain where event dispatch
should stop bubbling upwards.

Since event dispatch is done recursively, this was not working right,
as we would simply return from the innermost dispatch loop, leaving
the event un-accepted, which meant that the penultimately inner
dispatch loop would pick up the event and keep bubbling it anyway.

This made it possible for events to jump across window boundaries
within an application, in cases where one window was a CObject ancestor
of another window. This is typically the case with dialog windows.

Fix SerenityOS#1078.

Libraries/LibCore/CObject.cpp

@@ -189,6 +189,11 @@ void CObject::dispatch_event(CEvent& e, CObject* stay_within)
  do {
  target->event(e);
  target = target->parent();
+ if (target == stay_within) {
+ // Prevent the event from bubbling any further.
+ e.accept();
+ break;
+ }
  } while (target && target != stay_within && !e.is_accepted());
 }