move xml tag utilities to xml_tag.cc (#960)

move xml tag utilities to xml_tag.cc (#960)

* move xml tag utilities to xml_tag.cc

they are already declared in src/core/xmltag.h

* move xml_tag.cc to src/core, where it's h file is.

* move global xml_tag routines into class.

* make xml_next non-static.

* make xml_findnext non-static.

* make xml_findfirst non-static

* make xml_attribute non-static

* eliminate some shadowing

* rename XmlTag class.

* retire fs_xml_alloc.

* use QStringView with XmlTag.

* back off QStringView for xml_attribute for Qt 5.