Remove all the old 2.x and 3.x version annotations.
GTK+ 4 is a new start, and from the perspective of a
GTK+ 4 developer all these APIs have been around since
the beginning.
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.
