overview: Add a facility for swipe-scrolling

overview: Add a facility for swipe-scrolling

The workspaces view allows to drag the active workspace to swipe-scroll
to the next or previous workspace. While this behavior can come in handy
in general, there are good reasons to move the functionality to the
overview:

 - Finding a spot on a workspace to start a drag can be hard,
   especially when the workspace contains a single window
 - With the new layout, workspaces have no visible border, making
   it hard to predict where a drag can be initiated
 - The same behavior is equally useful for other elements

So add setScrollAdjustment() to the overview, which allows setting
an adjustment controlled with swipe-scrolling (either horizontally
or vertically); only a single adjustment can be controlled at a
time. A swipe-scroll can be initiated on any part of the background that
is not occupied by a reactive actor. For cases where further control
is needed, 'swipe-scroll-start' and 'swipe-scroll-end' signals are
emitted.

https://bugzilla.gnome.org/show_bug.cgi?id=635034