Improve unsetting old buffer in gtk_text_view_set_buffer()

Improve unsetting old buffer in gtk_text_view_set_buffer()

When unsetting the old buffer always set the buffer on the layout to
NULL.  More importantly, clear the pending scroll.  (The scroll is
handled in an idle, when not cleared an idle handler might touch the
layout later on, possibly corrupting the BTree).  Unref the buffer after
removing the selection from the clipboard, not before.  Patch merged
from maemo-gtk.