Made installation of tag file optional.

Made installation of Doxygen tag file optional in order to avoid an error
message if it has not been generated, e.g. because "make html" was not called
before "make install".
Added another addContentItem() method.

Added new addContentItem() method that expects a pointer to a newly created
document tree node. This provides another approach of creating the document
tree of an SR document, and avoids making the detour using getCurrentItem().