giscanner: Add a simple automatic sections file generator

This is a very basic sections file generator, and isn't too smart.
It's simply intended to be a base to build docs on, and will be used
if the user doesn't provide a sections file when calling g-ir-doc-tool,
for convenience purposes.

https://bugzilla.gnome.org/show_bug.cgi?id=699856
giscanner: Fix PEP-8 issue