Implement support for dynamic casting between QGstObject subclasses.

Implement support for dynamic casting between QGstObject subclasses.

This adds a new qtgstreamer_cast() template function that takes into account the internal
GType of the object it is casting and appropriately returns NULL if the cast cannot be done.
For example, casting a QGstElement to a QGstPad will fail, but casting a QGstPipeline to
QGstElement and back to QGstPipeline will succeed.