Make it clear that GetSheets() is heavy.

Also removes the side-effect that SCH_SHEET_LIST's
c'tor would sort the list (and write virtual page
numbers) anytime the starting sheet was the root.

Also, definitely don't build a SHEET_LIST (sorted or
otherwise) if you're not even going to use it.

Also don't build SCH_SHEET_LISTs on idle events.  Better
to just always have the Next Sheet button enabled (we
already beep if you click it and there's no next sheet).

Also, use a SCREEN_LIST when you can.  It's much cheaper
to create.

eeschema/sch_commit.cpp

@@ -567,7 +567,7 @@ void SCH_COMMIT::Revert()
                 {
                     // Lazy eval of sheet list; this is expensive even when unsorted
                     if( sheets.empty() )
-                        sheets = schematic->GetUnorderedSheets();
+                        sheets = schematic->BuildUnorderedSheetList();
 
                     SCH_SHEET_PATH sheet = sheets.FindSheetForScreen( screen );
                     symbol->SetRef( &sheet, field->GetText() );