* Encapsulate file name member of base screen object.
* Encapsulate associated screen member of schematic sheet object.
* Create add screen method to schematic sheet object to simplify setting
  the associated screen.
* Move the change file name code in the schematic sheet object to the edit
  sheet method in the schematic editor frame object to eliminate message
  dialogs.
* Improve reference counting in schematic screen object.
* Add helper type definitions for changing schematic object storage to C++
  containers.