Checking for a parent is not enough, it must have a toplevel one.
If widget has no toplevel parent then add it in a GtkWindow, that
make it usable from gst-launch-1.0.
https://bugzilla.gnome.org/show_bug.cgi?id=751104
This patch allow going gst-inspect-1.0 on these elements removing
ugly crash that was previously occurring. The method consist of
making the widget creation as lazy as possible. This way we don't
endup doing gtk_init() before the application. We also ref_sink()
the widget, so we don't crash if the parent widget is discarded,
and cleanly error out with GL if the widget has no parent window,
because calling gtk_widget_realized() can only be done if the widget
has been parented to a window).
