Store pointers to partition objects in TreeView_Details (#750168)

This stops copying of each displayed partition object into the
TreeView_Details class.

It also stops copy constructing lots of partition objects when just
clicking on a partition in the disk graphic.  The disk graphic needs to
inform the main GUI and then the partition list which partition has been
selected.  The call sequence goes like:

    DrawingAreaVisualDisk::on_button_press_event(event)
      Win_GParted::on_partition_selected(partition_ptr, src_is_treeview)
        TreeView_Detail::set_selected(partition_ptr)
          TreeView_Detail::set_selected(rows, partition_ptr,
                                        inside_extended)

Relevant source and highlighted comparison line:

   140  bool TreeView_Detail::set_selected( Gtk::TreeModel::Children rows,
   141                                      const Partition * partition_ptr, bool inside_extended )
   142  {
   143          for ( unsigned int t = 0 ; t < rows .size() ; t++ )
   144          {
>> 145                  if ( static_cast<Partition>( rows[t][treeview_detail_columns.partition] ) == *partition_ptr )
   146                  {
   147                          if ( inside_extended )
   148                                  expand_all() ;
   149
   150                          set_cursor( static_cast<Gtk::TreePath>( rows[ t ] ) ) ;
   151                          return true ;
   152                  }
   153
   154                  if ( set_selected( rows[t].children(), partition_ptr, true ) )
   155                          return true ;
   156          }
   157
   158          return false ;
   159  }

Then in this function the partition selected in the disk graphic
(partition_ptr parameter) is compared in turn with each partition object
stored in the Gtk::TreeView model to find the matching one to mark it as
selected.  This mere act of accessing the partition object stored in a
row of the Gtk::TreeView model causes it to be copy constructed.  So
clicking on the 5th partition in the disk graphic will copy construct
the first 5 partition objects just to do a compare to find the matching
one.

This is because it is not possible to get a reference from a
Gtk:TreeViewProxy in gtkmm.  Merely accessing a value in a Gtk::TreeView
model takes a copy of that value.

    Subject: get a reference from a Gtk::TreeValueProxy
    http://comments.gmane.org/gmane.comp.gnome.gtkmm/2217
    http://marc.info/?t=104400417500001&r=1&w=4

Bug 750168 - Reduce the amount of copying of partition objects