gtk.lisp: update prompt-buffer on mouse button events
gtk.lisp: update prompt-buffer on mouse button events

In the X11 tradition, mouse-2 pastes from the XA_PRIMARY clipboard, so
the prompt-buffer should update accordingly.

The prompt buffer will update regardless of which mouse button is
pressed, as we don't have a good way to decode if the user has
remapped their mouse buttons here, or if they have assigned custom
actions to other buttons

jmercouris/NeXT,jmercouris/NeXT