Implemented Gtk{Socket,Plug} in gtk/gtkx

[runjak]

• 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.

[jrick]

This will need !windows (and possibly !darwin) build tags so this package is not built on platforms without X11 when using go get ./...