Add Support for GetStructure in more messages types

[Philippe-Torrelli-Ale]

Hi, I wanted to use a "level " plugin in a pipe line.
This plugin sends Element messages with information in a structure.

Unfortunately Message.GetStructure only returns a Structure on Info, Error, and Warning Message but doesn't call
gst_message_get_structure for the other message types.

[danjenkins]

Try out #38

[Philippe-Torrelli-Ale]

Sorry for the delay,
@danjenkins I just tried it from your fork and it works perfectly.
Thanks a lot and sorry for having missed it was already an existing ticket.

[danjenkins]

Not mine - just noticed you asking for something I'd already seen a PR for.