GtkFileChooserDefault actually implements a binding signal for
Backspace, to make it go to the parent directory.  However,
GtkTreeView was eating our Backspace, and thus the file chooser was
not getting a chance to execute its binding signal.

GtkTreeView implements a Backspace binding itself, which it uses to
move to the parent node of the current cursor node.  However, the
binding handler would return TRUE even if there was no parent to the
current node.  Now the binding handler only returns TRUE if it
actually changed the cursor.

Additionally, gtk_tree_view_key_press() sees if no bindings handled a
key press; in that case, it re-sends the key press to the treeview's
search entry.  However, sending a Backspace to an empty entry makes
the entry beep.  Thus, we add a flag that gets set from GtkTreeView's
Backspace binding handler, to tell gtk_tree_view_key_press() when it
should *not* re-emit the key press on the search en...