Changing DisplayGroup assignment doesn't purge XMDS cache v2

[alexharrington]

https://community.xibo.org.uk/t/group-assignment-doesnt-trigger-xibo-client/5895/5

Issue remains after 1.7.8 initial release

https://github.com/dasgarner/xibo-cms/blob/b33f46440b0e6aba52f569158a46d939ca7956c0/lib/pages/displaygroup.class.php#L482

and

https://github.com/dasgarner/xibo-cms/blob/b33f46440b0e6aba52f569158a46d939ca7956c0/lib/pages/displaygroup.class.php#L501

maybe need to read:

PDOConnect::getPool()->getItem('display/' . $displayId);

With that patch in place it then appears to work as expected if you assign/unassign from the Display Groups page, but doesn't appear to work if you assign/unassign from the Displays page.