Add multipage example to pdf.pro; work with 5.14; fix main.cpp

- build the multipage example by default
- don't import anything that won't work with Qt 5.14
- set application attributes before creating an instance
  (to fix the warning about that)

Change-Id: I265f49ca75cae1908d4c23848cba8c42e5e3824b
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>