[Lazarus] GStreamer API unit added

[Michael Van Canneyt]

Hello,

For all people interested in sound/video streaming:

After lots of work, I added the gst unit to the FPC packages.
This unit contains the import declarations for the full libgstreamer-1.0 API. 
I didn't translate all C macros, but if you need some which I didn't
translate, let me know and I will add them.

For those that are not in the know:
gstreamer is a powerful audio/video stream system, used on most modern linux
distributions (it may be available on mac/windows, but I didn't test those).

I have also added a simple demo, that shows how to play a mp3 file.

I'm not an expert in sound/video streams, so if someone can contribute a
more elaborate example, it would be most welcome :)

Enjoy,

Michael.