Provide a macroId to uniquely identify multiple instances of a macro on a page

It's often useful to be able to distinguish multiple invocations of a macro on the same page, for instance in JavaScript used by the Macro. So a lot of macros implement some kind of macroId variable that's substituted into the HTML/JavaScript it outputs.

This is easy enough to implement, but it is duplicated across several plugins, with a variety of different implementations, and you'd be surprised how often people have trouble with it or just get it wrong (e.g., see CONF-12125, which is caused by a user macro that tries to use a random number as the macro id!)

If the framework passed a macroId to each macro and user macro, it would be a small but useful improvement to the plugin development experience.