Enable X mouse buttons 6 and 7 for horizontal scrolling.

[nbriggs]

Lisp code (WHEELSCROLL) now supports horizontal scrolling on sequences of PAD4 or PAD5 key presses (on the keyset)

This update enables passing mouse button 6 and 7 events through to Lisp on those keys.

[masinter]

I assume there's a reason to use Button5 + 1 instead of Button6.
Are there accompanying changes to KEYACTION ?

[nbriggs]

@masinter -- X does not have a Button6 or Button7 define, it stops at Button5. However, it produces values Button5+1 and Button5+2 for the two events we are interested in here.

Ron was making changes to KEYACTION when he and I last talked about this. Generally, I'd be happier if you'd approve (if appropriate) a PR, and I'll make sure that any other coordination (such as KEYACTION changes) has happened before I merge it.

[masinter]

That big green "Squash and Merge" button is just too attractive.
I realized after I did it.