action-row: Redesign child packing

By far the most commonly added kind of children to HdyActionRow are what we call action children. For this reason I would like to change how we pack its children:

• New actions should be packed after the previous ones, not before (we need to use a sub box and to pack actions at its start, not pack actions at the end of the header box).
• Packing actions shouldn't require a special child type, they should be the default way to pack children.
• Widgets expanding the row below it should not be the default but have a special type as they are likely the least common child type.

This would break both the API and the behavior of HdyActionRow and hence should be introduced with a hard library version change like libhandy-0.1.