cxxll::looks_like_xml: New template function
cxxll::maven_url: New class
cxxll::string_source: Add missing explicit to constructor
cxxll::vector_source: New class
cxxll::maven_url::extract(): Strip whitespace from URLs
cxxll::expat_source: Introduce proper exceptions

Avoid re-using std::runtime_error for XML errors.
cxxll::maven_url: Do not parse non-<project> XML files

Also ignore XML errors on the first element, except if it is an entity
declaration.
SQL schema: Add table symboldb.xml_error
database::add_xml_error(): New member function
SQL schema: Add table symboldb.java_maven_url

And enum type symboldb.java_maven_url_type.
cxxll::database::add_maven_url(): New member function
rpm_load: Extract URLs from Maven POM XML files
cxxll::expat_source: Deal with zero offset

XML_GetCurrentByteIndex can return 0 on certain errors.
cxxll::maven_url: Add invalid XML test

This used to crash in cxxll::expat_source from time to time.