Rename parameter to selected_partition in Dialog_Partition_New methods (#757671)

This is just to make the parameter name in the Dialog_Partition_New
constructor and set_data() method match the name of the equivalent
parameter in the Dialog_Partition_Copy and Dialog_Partition_Resize_Move
classes.  (All three classes inherit from Dialog_Base_Partition and have
similar interfaces).

Bug 757671 - Rework Dialog_Partition_New::Get_New_Partition() a bit