Add mapping for X11 generic events

[lafoletc]

Allow the reception of X11 Generic events.

https://www.x.org/releases/X11R7.5/doc/man/man3/XGetEventData.3.html

[matthiasblaesing]

Could you please point to the definition/headerfiles for the structures? That would help to review.

For the commit please check the author settings. The committer email looks sane, but for the author name only your firstname is set. To fix the existing commits, please squash them and update the author information while doing so.

Please also add an enhancement entry to the CHANGES.md entry. Have a look at the existing entries as template.

Thanks.

[dbwiddis]

The structures are already defined in X11 (Display is just a decorated PointerType), and match here.

Is it possible to create a test case to exercise these functions? Looks like sample code at the above manpage link.

[matthiasblaesing]

Merged via 8f17bdd

I finished the squash and fixed a remaining problem checkstyle complained about.

Thank you.