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.