renderer/gtk: Remove the buffer from the container it belongs to.
renderer/gtk: Remove the buffer from the container it belongs to.

Closes #2455.

The reason was that buffer belongs to main-buffer-container, which is located
too deep inside root-box-layout for gtk:gtk-container-remove to find the buffer
there. Thus, gtk:gtk-container-remove was destroying everything in the
root-box-layout, including the not-found-thus-not-deleted buffer view.

jmercouris/NeXT,jmercouris/NeXT