st-button: Handle touch events

On touch events, StButton becomes locked to a single sequence, so no multiple
touches can trigger it simultaneously.

https://bugzilla.gnome.org/show_bug.cgi?id=733633
popupMenu: dismiss the menu on touch events

No sequence checks are done, just any touch outside will dismiss the popup.

https://bugzilla.gnome.org/show_bug.cgi?id=733633
panelMenu: Interact to touch events

No sequence checks are done, these UI elements promptly trigger a grab that
will cancel ongoing touches and redirect later ones somewhere else, so that
works as a barrier to multi-toggling.

https://bugzilla.gnome.org/show_bug.cgi?id=733633
backgroundMenu: Allow for long presses on touch devices

These don't have a button set. Also, popup the menu relative to gesture
reported coordinates, and not relative to the pointer position everytime.

https://bugzilla.gnome.org/show_bug.cgi?id=733633
slider: React to touch events

The slider locks to the first interacting sequence, and ignores events from
any other.

https://bugzilla.gnome.org/show_bug.cgi?id=733633
appDisplay: Allow clicks/long presses through touch events

The long press code has been refactored so it can be used on both pointer and
touch events, and the click gesture has been made to account for button=0.

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