More consistent naming in help

This fixes some string issues in the help that occurred during renaming from Evince and Document Viewer to Papers:

• Changing Ev->Pps led to some strings where the word Even was changed to Ppsen, so change them back.
• Remove the in the <app>Papers</app>, it sounds a bit strange when Papers is a name. Only kept it when it is part of phrases like the <app>Papers</app> documentation
• Change some previously missed Document Viewer to Papers
• Remove duplication from the string Papers, formally known as Papers[...]