GeditDocument: remove comments explaining standard GObject code

And other small improvements to the header.
GeditDocument: remove useless #include's
GeditDocument: remove dead code
GeditDocument: save_metadata() static function

So dispose() is smaller.
GeditDocument: remove dispose_has_run attribute

doc->priv->location != NULL is sufficient.
GeditDocument: get_language_metadata() static function

To avoid code duplication.