usage: make file history's buttons shrinkable

The homogeneous on the boxes for the action buttons in the file history sub-panel has no visible effect, as the buttons lack hexpand. It, however, still influences the box's minimal width unnecessarily. Thus, removing it decreases the panel's minimal and natural width without a visible change otherwise.

Making those buttons can-shrink also decreases their and henceforth the panel's minimal width. With this change, the panel can shrink to a width of 360px, with a natural width not far from it.