port to gtk3

[v1cont]

i propose a series of patches for porting gst123 to gtk3. in additional to port code itself, there is a desktop file for opening files with gst123 in a xdg manner, and fix for working without control terminal

[swesterfeld]

Ok, I've merged most of your code into master. I didn't merge:

• code intended to make gst123 usable without controlling tty (or via .desktop file) - the reason is that in case of audio files, gst123 will not open a window, so the user will not have any option to stop playback - gst123 is intended as a command line player, if you want a full blown UI player, there are probably better solutions
• the possibility to increase volume to over 100% - this has always been supported, so I don't understand the point of your change