gstgtk: Allow doing gst-inspect-1.0 on these elements

gstgtk: Allow doing gst-inspect-1.0 on these elements

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).