XInput 2 support (again)

[Po Lu]

Some of the feedback I've been getting lately about the xwidget feature
is that scrolling in xwidgets is "choppy".  This seems to be because we
don't get pixel-wise scroll events from X, as we rely on the Core Input
API for input events.

I brushed up my XInput 2 patch from last month.  I would like to install
this as an optional feature if nobody has any immediate objections to
it, so I can begin work on adding support for better event handling in
xwidgets.  (This feature is entirely optional: if `--with-xinput2' is
not specified as an argument to configure, then the code will behave
exactly as it used to.)

Thanks.

0001-Add-support-for-event-processing-via-XInput-2.patch
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