keyboard: make the entire panel scrollable

The current Keyboard panel allows only the shortcut list to scroll.
Since we want to add more widgets above the list, and we want them
to scroll out of sight per the latest mockups [1], making only the
shortcut list scrollable poses a problem.

Fix that by making the entire panel scrollable.

[1] https://github.com/gnome-design-team/gnome-mockups/blob/master/system-settings/keyboard/keyboard-wires.png

https://bugzilla.gnome.org/show_bug.cgi?id=777840