sessionMode: Add support for `debugFlags` parameter

A session can now define `debugFlags` from a json file, while still leaving
priority to the environment variable.

This could be needed by distributions that want to provide debug modes or to
make possible to GDM to have specific flags on its mode.

All this without setting environment variables in .desktop exec scripts, but
making everything cleaner inside gnome-shell itself only.

https://bugzilla.gnome.org/show_bug.cgi?id=789377
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