Fix a crash on Shift-Right, expand all

It seems that removing the placeholder child during "test-expand-row"
causes an invalid iterator somewhere in gtk.  Instead, I now replace the
values of the placeholder directly with real first child, then continue
appending siblings as needed.

There's still a case which may remove, but only when there are no real
children, so "test-expand-row" returns TRUE to stop expansion.
Go back to a manual "query-tooltip"

The automatic GtkTreeView tooltip uses gtk_tooltip_set_markup, which
barfs on unescaped special characters, especially "<>" in C++ names.