Use a simple public MapInfo structure instead of trying to wrap it in a QSharedPointer class

Also add a test for my Memory classes
GstSample is a MiniObject type
Structure is a non-reference counted type of Gst Object.
Remove unnecessary g_free.

Also add a comment to indicate QGlib::Private::stringFromGCharPtr frees memory
DiscovererStreamInfo and DiscovererInfo are Objects not MiniObjects
PNGs are seekable in 1.0.

I double checked using the python Gst bindings.