tab-view: delay notify::selected-page until after setting
Currently, notify::selected-page can be emitted before the item is available in the wrapped GtkSelectionModel as it has not yet been notified. This causes NULL dereference when used with tooling such as a GtkListView.
To fix this, we need to set the selection internally, then emit the notify signal after the wrapped selection model has been updated.
Fixes #235 (closed)