Use G_DEFINE_xxx type macros in output files.

Use G_DEFINE_xxx type macros in output files.

The G_DEFINE_TYPE_EXTENDED and G_DEFINE_DYNAMIC_TYPE_EXTENDED can be
used to produce the default get_type implementations.  They produce
almost identical code to what we already generate in GOB, except that
we can now automatically pick up any improvements made in GLib.  For
example, with modern GLib the G_DEFINE_TYPE_EXTENDED will produce a
thread-safe get_type implementation which is a nice bonus.

These macros do not support the "prealloc" attribute so setting this
in gob will no longer do anything.  According to the documentation, this
has been a no-op in GLib itself since 2.10, and it appears to just be
for performance tuning anyway, so I doubt anyone cares.

The generated output will fall back to the original method when these
macros are not available.