Make it possible to update an extension to another extension point

[Nicklas Nordborg]

There is currently a problem if an update is made to a registered extension that includes moving the extension to a different extension point. This is mainly a problem with testing but may also affect real extensions. Here is the scenario:

1. An extension is developed and registered for a specific extension point, say A.
2. The extension is updated but we now want to register it under a different extension point, say B.
3. Installing the new xml/jar file is ok, we get a message that the extension has been updated.
4. The only problem is that the move from A to B is ignored. The extension is still registered with extension point A.

[Nicklas Nordborg]

(In [4875]) Fixes #1287: Make it possible to update an extension to another extension point

Moving to another extension point is considered as one de-registration and one new registration.