CA-141101: XenCenter filters not changeable after dismiss all - FIXED

- In the Events view, offer a choice only to dismiss the visible ones instead of all
- Keep the filter buttons always enabled. This applies to all Notification views (Alerts, Updates & Events)
- Also refined the confirmation messages displayed when dismissing events

Signed-off-by: Mihaela Stoica <mihaela.stoica@citrix.com>