Stop copying selected_partition back on itself in the copy dialog (#750168)

The code goes like this:

    Dialog_Partition_Copy::Get_New_Partition()
        call Dialog_Base_Partition::Get_New_Partition()
            Update this->selected_partition with results from running
            the dialog.
            return this->selected_partition by value.
        Save value back to this->selected_partition.
        Update this->selected_partition some more.
        return this->selected_partition by value.

So there is an unnecessary copy of the partition object returned from
the base class Get_New_Partition() function back to the same variable in
the derived copy class Get_New_Partition() function.

Need to keep the base class Get_New_Partition() function as derived
class Dialog_Partition_Resize_Move uses that implementation as it
doesn't override it, and it's part of the interface.

Avoid this unnecessary copy by moving base class Get_New_Partition()
code into a new private function, called prepare_new_partition(), which
doesn't return anything.  Then have Get_New_Partition() in both classes
just return the required partition object.  Like this:

    Dialog_Base_Partition::Get_New_Partition()
        call prepare_new_partition()
        return this->selected_partition by value.

    Dialog_Partition_Copy::Get_New_Partition()
        call Dialog_Base_Partition::prepare_new_partition()
        Update this->selected_partition some more.
        return this->selected_partition by value.

Bug 750168 - Reduce the amount of copying of partition objects

include/Dialog_Base_Partition.h

@@ -56,6 +56,8 @@ protected:
 		PASTE		= 2
 	};
 
+	void prepare_new_partition( Byte_Value sector_size );
+
 	void Set_Confirm_Button( CONFIRMBUTTON button_type ) ;
 	void Set_MinMax_Text( Sector min, Sector max ) ;
 

src/Dialog_Base_Partition.cc

@@ -137,6 +137,12 @@ void Dialog_Base_Partition::Set_Resizer( bool extended )
 }
 
 Partition Dialog_Base_Partition::Get_New_Partition( Byte_Value sector_size )
+{
+	prepare_new_partition( sector_size );
+	return selected_partition;
+}
+
+void Dialog_Base_Partition::prepare_new_partition( Byte_Value sector_size )
 {
 	//set sector size of new partition
 	selected_partition .sector_size = sector_size;
@@ -221,8 +227,6 @@ Partition Dialog_Base_Partition::Get_New_Partition( Byte_Value sector_size )
 	//if the original before value has not changed, then set indicator to keep start sector unchanged
 	if ( ORIG_BEFORE == spinbutton_before .get_value_as_int() )
 		selected_partition .strict_start = TRUE ;
-
-	return selected_partition ;
 }
 
 void Dialog_Base_Partition::Set_Confirm_Button( CONFIRMBUTTON button_type ) 

src/Dialog_Partition_Copy.cc

@@ -97,7 +97,7 @@ void Dialog_Partition_Copy::Set_Data( const Partition & selected_partition, cons
 	               , ceil( fs .MAX / double(MEBIBYTE) )
 	               ) ;
 
-	//set global selected_partition (see Dialog_Base_Partition::Get_New_Partition )
+	// Set member variable used in Dialog_Base_Partition::prepare_new_partition()
 	this ->selected_partition = copied_partition ;
 	this ->selected_partition .device_path = selected_partition .device_path ;
 	this ->selected_partition .inside_extended = selected_partition .inside_extended ;
@@ -118,7 +118,7 @@ void Dialog_Partition_Copy::Set_Data( const Partition & selected_partition, cons
 Partition Dialog_Partition_Copy::Get_New_Partition( Byte_Value sector_size )
 {
 	//first call baseclass to get the correct new partition
-	selected_partition = Dialog_Base_Partition::Get_New_Partition( sector_size ) ;
+	Dialog_Base_Partition::prepare_new_partition( sector_size );
 
 	//set proper name and status for partition
 	selected_partition .status = GParted::STAT_COPY ;