Simplify the public interface of Buffer::doExport

Simplify the public interface of Buffer::doExport

All calls to doExport from outside of Buffer have the same algorithm to
determine whether the included children parameter should be true. Moreover,
this decision is only based on information from the Buffer itself.

Now, Buffer only has 1 public function doExport and preview.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@40041 a592a061-630c-0410-9148-cb99ea01b6c8