applets/kicker: improve keyboard navigation without favorites

When there are no favorites, kicker still tries to assign focus there when using arrow key navigation, as if there were favorites. Similarly, having only one category of favorites leads to unusual arrow key navigation, as kicker still occasionally tries to focus the absent category.

This change improves the handling in these cases. To do this, we factor out common focus changing patterns into functions and call these; the existing code is also switched to these yielding shorter and more readable code. Along the way, we fix a few more edge cases, such as arrow down from the runner columns not working if the first column isn't Applications.