CP-6093: Disable filter buttons on the Alert and Events pages when the lsit is empty.

Signed-off-by: Konstantina Chremmou <konstantina.chremmou@citrix.com>
CP-6093: Removed top label and image from the Updates page. Show this info on the
progress panel instead. Some code simplification.

Signed-off-by: Konstantina Chremmou <konstantina.chremmou@citrix.com>
CP-6093: Anyone outside the Alert class should not know anything about the lock
used to access the alerts list. The list itself should be private as there is a
risk to access it and forgetting to use the lock.

Signed-off-by: Konstantina Chremmou <konstantina.chremmou@citrix.com>
CP-6093: Remove update alerts from the Alerts page, part 1/3: some code simplification
and efficiency improvements.

Signed-off-by: Konstantina Chremmou <konstantina.chremmou@citrix.com>
CP-6093: Remove update alerts from the Alerts page, part 2/3: Merged classes Updates
and ManualCheckForUpdates.

Signed-off-by: Konstantina Chremmou <konstantina.chremmou@citrix.com>
CP-6093: Remove update alerts from the Alerts page, part 3/3: Do not add updates
to the total Alert list.

Signed-off-by: Konstantina Chremmou <konstantina.chremmou@citrix.com>
CP-6093: Redesign of the CloseXenCenterWarningDialog to use the same controls as
the events page. This renders the old custom history controls obsolete, hence
removed them. Moved ActionBase extension methods to XenAdmin.Core namespace.

Signed-off-by: Konstantina Chremmou <konstantina.chremmou@citrix.com>
Unregister registered CollectionChanged event on disposal.

Signed-off-by: Konstantina Chremmou <konstantina.chremmou@citrix.com>