Revise pane_reparent()

Revise pane_reparent()

Treating a disconnected pane (p->parent == p) as a special cas in
pane_reparent() is unpleasant as disconnected panes are not something I
want.

So change pane_reparent() to have a different rule for determining
whether the new parent needs to be attached below p->parent first.

Also ensure Child-Notify notifications are consistant.

If the reparent fails, LOG an error and teach python interface to
trigger an exception.

Finally, only notmuch depended on the special meaning of disconnected
panes, so change it to fit the new model and add better explanations.

Signed-off-by: NeilBrown <neil@brown.name>