XInput 2 support

[Po Lu]

I hacked up support for handling XInput 2 events, though it hasn't
undergone any serious testing yet.

So the only advantage of this code over the existing Core Input code as
of present would be not relying on a fragile environment variable
(GDK_CORE_DEVICE_EVENTS) to work properly in GTK 3 builds.  Call me
paranoid, but this variable reeks of something that the GTK developers
will remove at some point in the future.  (AFAIK, it's already been
removed in GTK 4).

However, it paves the way for future support of high-resolution
scrollwheels, multi-touch trackpad support, and much more.

Would this support be worth installing?  Thanks.

0001-Add-support-for-event-processing-via-XInput-2.patch
Description: Text Data

(To test, configure with --with-xinput2 and make sure the XInput 2
development files are present on your computer.)