xinclude implicitely resolved

[mkraetke]

With this commit, XIncludes are automatically resolved. This may be practical in many situations, but sometimes you want to resolve the XInclude at a later point with <p:xinclude/>. Reasons for preferring an explicit XInclude resolution over an implicit are:

1. you want to speedup things by loading the documents at a later stage
2. one or more of the referenced documents are only available at a later point in the process

Perhaps we can make this feature optional?

[bwbohl]

Doesn’t expanding XIncludes automatically make the <p:xinclude/> somewhat superfluous?

In my case, I'm also relying on expanding the XIncludes at a later stage because I want to make some transformations on the included items first.