This is a GtkEventController implementation to handle mouse
scrolling. It handles both smooth and discrete events and
offers a way for callers to tell their preference too, so
smooth events shall be accumulated and coalesced on request.

On capable devices, it can also emit ::scroll-begin and
::scroll-end enclosing all ::scroll events for a scroll
operation.

It also has builtin kinetic scrolling capabilities, reporting
the initial velocity for both axes after ::scroll-end if
requested.