panel: Make the "Activities" button react to touch events

https://bugzilla.gnome.org/show_bug.cgi?id=733633
keyboard: Use common code to create regular and extension key buttons

The code is almost the same, so pull this out to a generic _makeKey(), and
use it from both places.

https://bugzilla.gnome.org/show_bug.cgi?id=733633
keyboard: Handle touch events

Handle touch events, so that an interacted button locks to a single sequence,
but multiple sequences are free to interact with multiple key buttons.

https://bugzilla.gnome.org/show_bug.cgi?id=733633