This adds support for the new wl_pointer events available in v5.

The wl_pointer.axis_source events can be ignored for the purposes here, the
main reason they exist is so that the combination of axis_source=finger and
axis_stop triggers kinetic scrolling. We don't need to care about the source,
axis_stop is enough for us to tell us when we're scrolling.

The wl_pointer.frame events group events together and is intended as a
mechanism to coalesce events together. This for example allows us to now
send a single GTK scroll event for a diagonal scroll. Previously, the two
wl_pointer.axis events had to be handled separately.

The wl_pointer.axis_discrete event sends mouse wheel clicks where
appropriate, and is translated into up/down/left/right scroll events.

https://bugzilla.gnome.org/show_bug.cgi?id=756729