This starts the process of working towards a new Libpeas 2.0 ABI. The
goals of this ABI break are:

 * Modernize and cleanup GObject usage within libpeas
 * Use GObject's in more places to allow for better GIO integration
 * Implement GListModel where it makes sense
 * Remove GTK 3 and GTK-based APIs as those are trivial to implement in
   applications with GTK 4 and newer.
 * Drop Python 2 support
 * Simplify support for new plugin loaders

# Conflicts:
#	NEWS
#	meson.build