Add support for extending multiple extension points at the same time

[Nicklas Nordborg]

The current extension system allows a single extension to extend a single extension point. But we also define some generic extension points (eg. net.sf.basedb.clients.web.toolbar.list.all) which makes it possible for extensions to appear in more than one place.

But if none of the generic extension points are suitable an extension must be defined multiple times to be used on multiple extension points. It would be better to allow a single extension to extend multiple extension points in the first place.

My idea is that as an alternative to specify the extension point as an attribute in <extension id="..." extends="..."> we should support something like this:

<extension id="..">
  <extends>
    <ref index="..">....</ref>
    <ref index="..">....</ref>
  </extends>
  ...
</extension>

The rest of the extension definition remains the same and it is of course assumed that all given extension points are using a compatible Action.

Note that this is a change to the XML format only. Internally multiple extension objects will be created.

[Nicklas Nordborg]

(In [5519]) Fixes #1554: Add support for extending multiple extension points at the same time

[Nicklas Nordborg]

Changed the XML example to show how it was actually implemented.