Migrate from PyGTK to PyGObject and GTK+3

[ockham]

Migrate from PyGTK to PyGObject (with Introspection), and GTK+ 3. This includes:

• Migrate our home-brewn preferences system to GSettings.
• Use GIO for file system related stuff.
• Use cairo instead of GDK.. Bonus points if PIL also gets replaced by cairo (that's issue Use cairo instead of PIL #706).
• Use libpeas for plugins.
• Instead of storing data in a hard-wired ~/.gourmet directory, use glib.get_user_config_dir () and glib.get_user_data_dir () or glib.get_user_cache_dir (), respectively.
• Use libstarred instead of our home-brewn starring system (particularly if it gets included into core GTK+, though that seems rather unlikely according to discussions with GTK+ on IRC).

Ideally, that code should get first released once there's a runtime available not only for Linux, but at least also for Windows, and ideally also for OS X. (Note that I've suggested PyGObject/Introspection Windows builds as a Gnome GSoC project idea.) The bits that don't require PyGObject/Introspection and GTK+3 (like the GDK->cairo migration) can be merged sooner, of course.

This is one of the ideas we're suggesting as a project for GSoC 2013. For general information about Gourmet's participation in GSoC 2013, see the wiki page.

[ockham]

Good news: there's now a Windows port of PyGObject and Gtk+3.