XInput 2.0 branch ready for review

[Carlos Garnacho <carlosg gnome org>]

Hi all!

I think The XInput 2.0 branch is ready for initial review, there are
some things missing, but at least the GDK part looks quite consistent,
there are some glitches in non-pure GTK+ apps, but as far as I've tested
in my jhbuild environment, it works quite well.

The patch [1] is fairly big (22'5K lines) and does quite heavy
modifications on pretty basic GDK internals, so I'm not sure how could
we best do for merging, I've tried to keep things working along git
history though.

I've also updated the MPX page in l.g.o [2] with some implementation
details and roadmap.

There are some things left, such as DnD or modifying GtkWidgets, but I
think that can be done step by step without disrupting anything. I'll
keep working on the missing items in the xi2 branch anyway.

So, questions? petitions? :)

Cheers,
  Carlos

[1] https://bugzilla.gnome.org/show_bug.cgi?id=596725#c1
[2] http://live.gnome.org/GTK+/MPX