Fix crash when changing Pane widgets -- #7198

Fix crash when changing Pane widgets -- #7198

Gtkmm2Ext::Pane::on_add() uses a pointer to a std::vector<> element
in the destroy notify callback. If the vector is modified, that pointer
becomes invalid.

Add 2 widgets "A", "B". remove "B", add another one "C".
Now if A is destroyed, notify_child_destroyed(PTR) points to
invalid memory and not to "A".