Update merge-activities to become a "canonical export" filter

[rolfkleef]

The current merge-activities already does some sorting (activities by activity identifier, dates by type, for example).

It can be extended to do some more sorting, to create a form of "canonical serialisation" of IATI data, with indenting, sorting, and so on.

This will make it easier to use conventional text-based tools for diffs, version management, and so on.

[rolfkleef]

Also check including attributes and elements from other namespaces.

Attributes should already be included?

It should be possible to also add additional xslt files with known extensions (maee, akvo), so that the generic version can just apply templates to anything in other namespaces.