Merged the PolicyHistory control into the dialog; removed legacy column from
the policies datagridview; put retrieval of messages on a background thread
to avoid UI freezing if there is a huge number of messages; retrieve messages
only when the VMSS collection changes and not every time the run history grid
is repopulated; sorted out button enabled state.
Signed-off-by: Konstantina Chremmou <konstantina.chremmou@citrix.com>
