Mark macro executions as isolated for better performance

When macros are executed, until now, the whole XDOM tree needed to be re-scanned for the next macro to execute. This led to slow rendering times for pages with thousands of macros. It is now possible to mark the execution of macros and wiki macros as isolated if their execution won't modify the XDOM, which avoids this costly scan. As this can significantly improve the rendering time, e.g., from eight to one second (concrete improvements depend on the exact page content), macro developers are advised to mark their macros as isolated if their execution is actually isolated.