keyboard: fix list sizes with latest Gtk+

Gtk+ changed again the behavior of the scrolled window,
fixing the weird height issue but introducing a new issue
where the keyboard shortcut list doesn't expand horizontally
nor allocates the correct size vertically.

Fortunately, this is easily solvable with the newly introduced
GtkScrolledWindow:propagate-natural-width and -height
properties.

Fix the misbehavior by setting the new properties to sane
values.

https://bugzilla.gnome.org/show_bug.cgi?id=769314