Schematic object encapsulation and other minor improvements.

* 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.

common/common_plot_functions.cpp

@@ -487,7 +487,7 @@ void WinEDA_DrawFrame::PlotWorkSheet( PLOTTER* plotter, BASE_SCREEN* screen )
         case WS_FILENAME:
         {
             wxString fname, fext;
-            wxFileName::SplitPath( screen->m_FileName,
+            wxFileName::SplitPath( screen->GetFileName(),
                                    (wxString*) NULL,
                                    &fname,
                                    &fext );