wayland: Move additional pointer buttons after the old 4-7 scrolling ones

wayland: Move additional pointer buttons after the old 4-7 scrolling ones

We were using that range for the extra buttons after left/right/middle,
while this is harmless for clients not handling extra buttons (we
used to translate those button events into scroll events in x11 anyway)
this will be unexpected for clients that do handle additional mouse
buttons themselves (eg. back/forward buttons present in some mice).

In order to remain compatible with X11, those need to be assigned from
button 8 onwards.

Also, include input.h, and stop using magic numbers here.

https://bugzilla.gnome.org/show_bug.cgi?id=758072