qdoc: Add a logging category

Remove the debug functionality from the Generator class and add
a logging category instead. Add some debug statements outputting
the command line arguments and clang arguments.

This makes it possible to obtain logging output by setting for
example QT_LOGGING_RULES=qt.qdoc.debug=true .

Task-number: PYSIDE-363
Change-Id: I3ecfe548e14aa3e2d03f19d07fc61a2647b75111
Reviewed-by: Martin Smith <martin.smith@qt.io>