[X11] Added XGetAtomName() function
[X11] Check before using motion/scroll valuators

These valuators are not available on all systems. We now check
explicitly for support, instead of returning invalid results.

Additionally, unknown valuator names are now printed to the
debug log.
[X11] Added strong-typed enum for XIValuatorInfo.mode
[X11] Fixed absolute motion valuators
[X11] Fixed core X11 scrolling and absolute y motions
Merge pull request #164 from thefiddler/x11_scrollfix

[X11] Fix legacy scrolling and absolute pointer motion