gst-src: Download and build GTK+

* gst-src/Makefile: Likewise.
maint: Syntax cleanup

* version.c: Likewise.
maint: Add dual.c and probe.c

* dual.c: New sample.
* probe.c: New sample.
* gstx.h: New file; Helper macros for Gstreamer.
* Makefile: Likewise; generate gdb.cfg and source.me.
* .gitignore: Updated accordingly.