Implemented Gtk{Socket,Plug} in gtk/gtkx

- In C GtkSocket and GtkPlug are used with '#include <gtk/gtkx.h>',
  so I figured it would make sense for them to have a similar module in go.
- There are two examples, selfplug and xterm,
  which demonstrate the Socket/Plug mechanism.
- The selfplug example is just a stack of window->socket->plug->button,
  to see that it's possible to have the Socket and the Plug both from the same program.
- The xterm example executes xterm behind the scenes,
  using the -into parameter to embed it into a Socket.
  Sadly in my test setup xterm wouldn't react to input.
Please use conformal import paths, even for your own changes.  It makes it easy to merge back into mainline gotk3.  You can do this by adding other git remotes and pushing your changes to you own github fork, while still keeping master in sync with upstream.