Fix action row focus behavior
Follow-up on !540 (closed)
Fixes #215 (closed)
Adds a focus handler to HdyActionRow, which delegates to its activatable subwidget (if it has one). This fixes the surprising tab focus handling. Now, pressing tab will either directly focus the HdyActionRow activatable subwidget (if it has one) or skip over it.
Note that the new behavior prevents tabbing over any other activatable subwidgets, such as the toggleable expander row shown in the example demo. I'm looking into how to handle those. I think ideally, tab focus should flow to the sub-components that aren't the activatable, but I'm not 100% sure how to do that yet.