LibGUI: Notify widgets when their enabled state changes.

This is done by dispatching a (synchronous) "EnabledChange" event that can
be picked up in change_event(). Use this event to kick widgets out of their
"being pressed"-type modes if the user is interacting with them while the
state is programmatically changed.

LibGUI/GScrollBar.cpp

@@ -300,3 +300,12 @@ void GScrollBar::leave_event(CEvent&)
         update();
     }
 }
+
+void GScrollBar::change_event(GEvent& event)
+{
+    if (event.type() == GEvent::Type::EnabledChange) {
+        if (!is_enabled())
+            m_scrubbing = false;
+    }
+    return GWidget::change_event(event);
+}