Add context to locale source strings

when exporting strings there is no contextual information associated with them which can make it more difficult for translation.

This patch adds a context-group and associated context tags to locale source strings and xliff export. Not a very clean patch but it's a start.

before

    <trans-unit id="1191][singular" resname="1191][singular">
     <source xml:lang="en">‹ previous</source>
     <target xml:lang="en-gb"/>
     <note>Singular</note>
    </trans-unit>

after

    <trans-unit id="1191][singular" resname="1191][singular">
     <source xml:lang="en">‹ previous</source>
     <target xml:lang="en-gb"/>
     <note>Singular</note>
     <context-group purpose="information">
      <context context-type="x-path">/fr/articles</context>
     </context-group>
    </trans-unit>