CP-6093: Tidying up: moved property from Alert to XenServerPatchAlert as it only
applies to the latter. Converted methods from AlertSummaryPage to one Alert and
one String extension method so they can be used more widely. Code simplification
on the Updates page.

Signed-off-by: Konstantina Chremmou <konstantina.chremmou@citrix.com>
Clear selected nodes at the beginning of each of the MainWindowTreeTests to prevent
test failures due to artifacts leftover from a previous test.

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