Stop copying selected partition object in Manage Flags dialog (#750168)

When opening the Manage Flags dialog, creation of the dialog object was
creating a copy of the selected partition object.  If this was an
extended partition it also included recursively constructing the
contained logical partitions too.

Instead, replace the partition object in the DialogManageFlags class
with a reference to it.

Bug 750168 - Reduce the amount of copying of partition objects