Fix interpreter not being set to python3

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Add tool to generate source code of accessors

Add `gobjectaccessor`, a tool to generate source/header code of the
accessors for a property of a GObject/GBoxed.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Add --version option to tools

The new command line option --version has been added to gobjectogen and
gobjectaccessor.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>