add "repopulate" method for view transitions

The "populate" transition for view transitions is only triggered when the view is first displayed or when the view's model changes. It would sometimes be useful to be able to trigger this transition manually; for example, when a view moves off-screen, and then back on-screen, it would be a good time to re-run the populate transition.

This could be achieved by adding a repopulate() function to ListView and GridView that re-runs the populate transition.