Move Eeschema globals to new SCHEMATIC object

Set up a new lineage for SCH_ITEMS to get back to the SCHEMATIC
they live on: Items will all be parented to the SCH_SCREEN that
they are added to, and each SCH_SCREEN will point back to the
SCHEMATIC that it is part of.  Note that this hierarchy is not
the same as the actual schematic hierarchy, which continues to
be managed through SCH_SHEETs and SCH_SHEET_PATHS.

eeschema/dialogs/dialog_plot_schematic.cpp

@@ -29,10 +29,10 @@
 #include <eeschema_settings.h>
 #include <kiface_i.h>
 #include <pgm_base.h>
-#include <sch_sheet.h>
 #include <settings/settings_manager.h>
 #include <ws_painter.h>
 #include <sch_painter.h>
+#include <schematic.h>
 
 // static members (static to remember last state):
 int DIALOG_PLOT_SCHEMATIC::m_pageSizeSelect = PAGE_SIZE_AUTO;
@@ -151,7 +151,7 @@ void DIALOG_PLOT_SCHEMATIC::OnOutputDirectoryBrowseClicked( wxCommandEvent& even
 
     wxFileName      dirName = wxFileName::DirName( dirDialog.GetPath() );
 
-    wxFileName fn( Prj().AbsolutePath( g_RootSheet->GetFileName() ) );
+    wxFileName fn( Prj().AbsolutePath( m_parent->Schematic().Root().GetFileName() ) );
     wxString defaultPath = fn.GetPathWithSep();
     wxString msg;
     msg.Printf( _( "Do you want to use a path relative to\n\"%s\"" ), GetChars( defaultPath ) );