grid-view: Restore 2-dimensional navigation from sushi

We used to rely on the "move-cursor" keybinding signal to forward arrow key movements from sushi to our files view.

But GtkListBase-derived widgets do not provide a replacement for that keybinding signal. While we have a good enough solution for list view (which is one-dimensional, so we can just iterate the list), it's a regression for grid view (up and down do not jump to next line, because the grid's layout is unknown to us).

As a workaround, poke into GtkListBase's internal keybindings and trigger their action programatically.

Resolves: #2556 (closed)