Merge overlapping operations (#438573)

When a new operation is added to operations list, check if a merge
is possible depending on the operation type:

    OPERATION_RESIZE_MOVE:  2 consecutive "resize" operations on the
                            same  partition
OPERATION_LABEL_PARTITION:  2 "label change" operations (need not be
                            consecutive) on the same partition
          OPERATION_CHECK:  2 "check" operations (need not be
                            consecutive) on the same partition
         OPERATION_FORMAT:  2 consecutive "format" operations on the
                            same partition

Closes Bug #438573 - Cancel out overlapping actions

Also fix a bug when copying partition using the Partition::Set(...)
method.  This method did not initialize "sectors_used" and
"sectors_unused" members.