gdbus-codegen: Take into consideration MAX_ALLOWED for private data

The G_ADD_PRIVATE() macro, and the auto-generated get_instance_private()
internal function, should be used conditionally depending on the maximum
allowed version of GLib, as defined by the GLIB_VERSION_MAX_ALLOWED
pre-processor symbol.

This allows generating code that can be compiled in projects that wish
to use an older API version of GLib through the use of the
GLIB_VERSION_MAX_ALLOWED symbol.

https://bugzilla.gnome.org/show_bug.cgi?id=710133
gdbus-codegen maintains ABI for type structures

Make it explicit, to avoid changes that could potentially lead to
breakage in user code.

https://bugzilla.gnome.org/show_bug.cgi?id=710133
docs: Mention that generated code should not be kept

Code generated by gdbus-codegen should neither be checked in into
revision control, nor should be distributed.

https://bugzilla.gnome.org/show_bug.cgi?id=710133