Yelp viewer can't open cross-references to other parts of
the manual using link linkend.

-= Validate =-
  wget https://docbook.org/xml/5.0/rng/dbits.rng
  xmllint --noout --relaxng dbits.rng help/C/index.docbook
  jing dbits.rng help/C/index.docbook

-= View the manual using yelp =-
  yelp file:///path_to_file/index.docbook

-= Tests: Generate pdf and html files from docbook =-

  Method I: Use yelp-build (HTML chunk)
    cd help/C
    mkdir htmlout
    yelp-build html -o htmlout index.docbook

  Method II-a: Use docbook5-style-xsl package on Fedora 29 (PDF)
    sudo dnf install -y docbook5-style-xsl
    cd help/C
    xsltproc /usr/share/sgml/docbook/xsl-ns-stylesheets-1.79.2/fo/docbook.xsl index.docbook > pluma.fo
    fop -fo pluma.fo -pdf pluma.pdf

  Method II-b: Use docbook5-style-xsl package on Fedora 29 (HTML)
    sudo dnf install -y docbook5-style-xsl
    cd help/C
    xsltproc /usr/share/sgml/docbook/xsl-ns-stylesheets-1.79.2/html/docbook.xsl index.docbook > index.html

  Method II-c: Use docbook5-style-xsl package on Fedora 29 (HTML chunk)
    sudo dnf install -y docbook5-style-xsl
    cd help/C
    xsltproc --xinclude \
        --stringparam base.dir htmlout/ \
        --stringparam use.id.as.filename 1 \
        --stringparam chunker.output.encoding UTF-8 \
        /usr/share/sgml/docbook/xsl-ns-stylesheets-1.79.2/html/chunk.xsl index.docbook
or
    xsltproc --xinclude \
        --stringparam base.dir htmlout/ \
        --stringparam use.id.as.filename 1 \
        --stringparam chunker.output.encoding UTF-8 \
        --stringparam chunk.section.depth 2 \
        --stringparam chunk.first.sections 1 \
        /usr/share/sgml/docbook/xsl-ns-stylesheets-1.79.2/html/chunk.xsl index.docbook
    cp -rp figures htmlout

  Method III-a: Use docbook-xslt2-2.3.10-app (XHTML)
    cd help/C
    wget https://github.com/docbook/xslt20-stylesheets/releases/download/2.3.10/docbook-xslt2-2.3.10-app.zip
    unzip docbook-xslt2-2.3.10-app.zip
    java -jar docbook-xslt2-2.3.10-app/docbook-xslt2-2.3.10.jar -f xhtml index.docbook > index.html

It doesn't work: Method III-b: Use docbook-xslt2-2.3.10-app (PDF)
    cd help/C
    wget https://github.com/docbook/xslt20-stylesheets/releases/download/2.3.10/docbook-xslt2-2.3.10-app.zip
    unzip docbook-xslt2-2.3.10-app.zip
    java -jar docbook-xslt2-2.3.10-app/docbook-xslt2-2.3.10.jar -f foprint index.docbook > pluma.fo
    fop -fo pluma.fo -pdf pluma.pdf

To upgrade the manual:
  sudo dnf install -y docbook5-schemas
  xsltproc --output index-new.docbook /usr/share/xml/docbook5/stylesheet/upgrade/db4-upgrade.xsl index.docbook
  xsltproc --output legal-new.xml /usr/share/xml/docbook5/stylesheet/upgrade/db4-upgrade.xsl legal.xml

To validate the manual:
  xmllint --noout --relaxng /usr/share/xml/docbook5/schema/rng/5.0/docbook.rng help/C/index.docbook
  jing /usr/share/xml/docbook5/schema/rng/5.0/docbook.rng help/C/index.docbook
  yelp-check validate help/C/index.docbook

To view the manual:
  yelp file:///full_path/index.docbook

Note: docbook5-schemas should be installed in your system in order to view the manual (DEPENDENCY)

Closes #395

Example of generated file:

help/pluma.pot before:

msgid "To report a bug or make a suggestion regarding the <application>pluma</application> application or this manual, follow the directions in the <ulink url=\"help:mate-user-guide/feedback\" type=\"help\">MATE Feedback Page</ulink>."
msgstr ""

help/pluma.pot after:

msgid "MATE Feedback Page"
msgstr ""

msgid "To report a bug or make a suggestion regarding the <application>pluma</application> application or this manual, follow the directions in the <_:ulink-1/>."
msgstr ""