Drop horizontal scroll mouse button events on Linux

[neilcsmith-net]

Drop horizontal scroll mouse button events on Linux to fix issues reported with trackpads triggering code navigation. The JDK already drops the X event button number by 2 to skip vertical scroll. This skips the horizontal scroll button values and drops the value another 2 before passing to the keymap, which should also bring Linux in line with other OS button values.

This is an alternative to #6636

Added a build for easier testing. Ideally we should check that Windows and macOS don't need the button 6&7 mappings anywhere.

[lkishalmi]

Well, this could work. My only concern is that mouseButtonKeyCode is a public API, we shall notify the users in Javadoc about the additional 4,5 virtual buttons for horizontal scrolling on Linux, so people would be aware of that.

[neilcsmith-net]

Funnily enough, my preference for handling here is partly simplicity, but also because I think it keeps the API as intended.

The mapping of hardware button to OS button to JDK button to NB windows system button is in many ways an implementation detail. In future we could make this change optional or even allow other options in the window system without changing the API.

[lkishalmi]

I agree on this approach. Just thought that some additional documentation on the API would be nice.

[mbien]

makes sense to me

[neilcsmith-net]

Thanks @lkishalmi Does the added implementation note documentation help? It's a bit weird to work out how best to document this, apart from being clear that doing this aligns with the intended API usage.

[lkishalmi]

Looks good now. Thank you!