include all translated strings

This way, the English translation file can be used as a reference to find missing translations in all other languages.

Author: xabbuh

app/Resources/translations/messages.en.xliff

@@ -18,6 +18,14 @@
  <source>mit_license</source>
  <target>MIT License</target>
  </trans-unit>
+ <trans-unit id="previous">
+ <source>Previous</source>
+ <target>Previous</target>
+ </trans-unit>
+ <trans-unit id="next">
+ <source>Next</source>
+ <target>Next</target>
+ </trans-unit>
 
  <trans-unit id="title.homepage">
  <source>title.homepage</source>