CA-140335: Fixed the problems with dismissing alerts

Changes to the DeleteAllAlertsAction:
- when the dismiss operation fails with INVALID_HANDLE we need to remove the alert from our alert list (XenCenterAlerts) and trigger the CollectionChanged event so that the alert count gets updated.
- the action title now says "Removing <n> alerts" when dismissing all/selected alerts and "Removing alert" when dismissing single alert (instead of "Removing all alerts" in all cases)
- move the initialization of Dismissing flag outside the action (see below)

This fix also includes changes so that the Dismissing flag works as intended: to be able to hide the dismissing alerts in the Alerts view immediately, rather than one by one when they were actually deleted, causing a large number of refreshes.
On dismissing alerts we do:
1. set dismissing to true for all alerts
2. rebuild alert list (this will filter out the dismissing alerts)
3. dismiss alerts - run the DismissAllAlerts action

Also, the action buttons on each row should only apply to the clicked row (not multiselect).

Signed-off-by: Mihaela Stoica <mihaela.stoica@citrix.com>
Merge pull request #146 from MihaelaStoica/CA-140335

CA-140335: Fixed the problems with dismissing alerts