Add generic templates (#307)

- Add generic full-page templates.  (This feature from 3.4 was missing)
- Also add generic half-page, quarter-page, and envelope templates.
- Add several more common envelope sizes to page-sizes.xml
- Qt print backend does not need pwg_size.
    - Replace with pwg_class, to distinguish between ISO and NA sizes

model/Db.hpp

@@ -114,6 +114,8 @@ namespace glabels::model
                 static void readTemplatesFromDir( const QDir& dir );
                 static void registerTemplate( const Template& tmplate );
 
+                static void createGenericTemplates();
+
                 static void readUserTemplatesFromDir( const QDir& dir );