LibCore+LibGUI+WindowServer: Make events bubble up through ancestors

With this patch, CEvents no longer stop at the target object, but will
bubble up the ancestor chain as long as CEvent::is_accepted() is false.

To the set accepted flag, call CEvent::accept().
To clear the accepted flag, call CEvent::ignore().

Events start out in the accepted state, so if you want them to bubble
up, you have to call ignore() on them.

Using this mechanism, we now ignore non-tabbing keydown events in
GWidget, causing them to bubble up through the widget's ancestors. :^)

Libraries/LibGUI/GAbstractButton.cpp

@@ -134,8 +134,11 @@ void GAbstractButton::leave_event(CEvent&)
 
 void GAbstractButton::keydown_event(GKeyEvent& event)
 {
-    if (event.key() == KeyCode::Key_Return)
+    if (event.key() == KeyCode::Key_Return) {
         click();
+        event.accept();
+        return;
+    }
     GWidget::keydown_event(event);
 }