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/CMakeLists.txt

@@ -31,6 +31,7 @@ set (Model_sources
   FramePath.cpp
   FrameRect.cpp
   FrameRound.cpp
+  GenericTemplate.cpp
   Handle.cpp
   Layout.cpp
   Markup.cpp