[viewport] support for horizontal scrolling with the mouse wheel(s)

[UnseenBook]

Now that horizontal scrolling is added to the viewport (thanks for that, it's great!) I would like to scroll horizontally with the mouse wheel. In some apps like image viewers you can horizontally scroll by holding shift when scrolling the mouse wheel. This would make it possible for everybody to horizontally scroll with the mouse wheel. But there are also mice that have a dedicated horizontal mouse wheel (like the Logitech MX Master 3S). For this one it's trivial to add support for horizontal scrolling with the mouse wheel.

I have prepared a PR implementing this idea. In the PR I decided not to add a HorizontalMouseWheelDelta, since there is already horizontalStep. I can image that not everybody agrees with this, I would like to hear what other people think of this.

While testing this change out I noticed that my terminal KDE Konsole does not send events for the horizontal mouse wheel and not even the Shift event for mouse actions to terminal apps. Some other terminals I tried did send these events and the scrolling worked with the mouse wheels. The terminals I tried were Konsole, Alacritty, Kitty, Tilix, and gnome-terminal and only in Konsole it did not work.

My PR: #761

[caarlos0]

#761 is merged! thanks @UnseenBook 🙏🏻