Cache full schematic sheet list sorted by sheet page number.

The sheet list is returned as a copy of the cached list rather than a
reference to prevent external code from changing the list.  While not as
performant, it eliminates the risk of the sheet list being altered in
ways that could break the schematic.  The sheet list should only be
updated by calling SCHEMATIC::RefreshHierarchy() when any appropriate
sheet changes are made.

Note to developers: there is something inherently different about how the
QA tests are loading and handling schematics versus the schematic editor.
Using the cached sheet list for the SCHEMATIC object will cause some QA
test to fail.  This is why SCHEMATIC::Hierarchy() has not replaced
SCHEMATIC::BuildSheetListSortedByPageNumbers() everywhere.

eeschema/sch_sheet.cpp

@@ -297,7 +297,7 @@ bool SCH_SHEET::ResolveTextVar( const SCH_SHEET_PATH* aPath, wxString* token, in
     }
     else if( token->IsSameAs( wxT( "##" ) ) )
     {
-        *token = wxString::Format( wxT( "%d" ), (int) schematic->BuildUnorderedSheetList().size() );
+        *token = wxString::Format( wxT( "%d" ), (int) schematic->Hierarchy().size() );
         return true;
     }
     else if( token->IsSameAs( wxT( "SHEETPATH" ) ) )