"Stick-to-finger" gesture behavior

High-level issue for the ideal behavior we'll eventually want for the continuous stick-to-finger gestures in GNOME Shell. Some parts of this are already implemented, some only require small changes, others might take a while to happen, but it would be good to have an overall plan for where we're going with this.

In general, all gestures should

• stick to fingers and follow gestures continuously
• have momentum
• have rubber-band feedback on the first/last page
• have a snap-back threshold at about half the height of a page (somewhat dependent on momentum)

As for the individual gestures, this is my proposal:

On the desktop:

• 4 finger gesture to move between workspaces
• 3 finger pinch to open the overview
• 4 finger pinch to open the overview

In the overview:

• 4 finger gesture to move between workspaces
• 2 finger gesture that behaves exactly like the 4 finger one
• 3 finger pinch to go back to the desktop
• 4 finger pinch to go back to the desktop

In the applications grid:

• 4 finger gesture to move between app pages
• 2 finger gesture that behaves exactly like the 4 finger one

cc @carlosg @aday @jimmac