Clearer message when no app backends present.

See issue #582
Add PyQt5 backend.

Luke already did most of the work for this. I only added
the necessary backend module.

Someone should test this (on Windows).
Make PyQt5 work. Also allow other Qt backends on QtOpenGL importerror
fix typo
PyQt5: fix scrolling and fix a test
Merge pull request #583 from almarklein/clearer-app-msg

Clearer app msg and pyqt5 backend