Allow to override tag methods

[scharf]

For example, it is not possible to override the update method, because the update method is not only writable:false (which makes sense to prevent accidental overriding) but also configurable:false which is an annoying restriction!

We have a mechanism to track the model objects that are accessed during an update. And we have a mechanism to call update if any of the variables used by update has changed. To implement that correctly, we need to override update.

I thought I could use the update and updated event track when an update is entered and exited, but that seems not possible.

[scharf]

All I ask for is to make the properties configurable:true ;-)