listbase: Select via action

listbase: Select via action

Instead of directly calling select_item(), trigger the select-item
action of the focused child.

We do this convoluted calling into the widget because that way
GtkListItem::selectable gets respected, which is what one would expect.

Plus, this code is usually triggered via keybindings, and this way the
ListBase keybindings work identical to the ListItem keybindings.