build: Require GLib 2.32 or above

GLib 2.32 has been released in March 2012, so we can now require it a
build and run time. This allows us to remove GLib 2.31 conditionals,
mark the Thread.Init as obsolete, and remove all references to
libgthread.

Please note that the API exposed in glib and gio is still from 2.28, but
we will have some API additions in the future. First example is the next
commit, which brings GBytes, an addition in GLib 2.32.
glib: Add binding for GBytes

This patch adds bindings for GBytes https://developer.gnome.org/glib/stable/glib-Byte-Arrays.html

The code was first generated using gobject-introspection and then
refactored. The new type is also added to the generator SymbolTable.