events: Bypass wayland when scrolling with mouse-button modifier pressed

is considered a system modifier, and applications cannot use it for keyboard shortcuts or as button modifier. It doesn't seem too much of a loss taking +scroll-event away as well, so that it becomes available to gnome-shell/extensions.

Disclaimer: I've only tested this in nested mode (with both wayland and X11 clients). I do expect this to work in a regular wayland session as well, but I'm not so sure about an Xorg session ...