synaptics rework - multiple frontends

[Peter Hutterer]

Jonas posted some touchpad support patches earlier this month and
there were some calls for a input handling module. I've hacked around for a
few days to get the synaptics driver ready for wayland.

This is not a wayland touchpad driver! It's the X11 driver, abstracted
enough that a wayland frontend _may_ be possible, if this is what we want.

Code is available here:
http://cgit.freedesktop.org/~whot/xf86-input-synaptics/log/?h=abstract-x-interface

Of particularly interest is the new dummy synaptics driver:
http://cgit.freedesktop.org/~whot/xf86-input-synaptics/tree/src/synaptics-dummy.c?h=abstract-x-interface
That file is 150 lines long and all it does is print events to the log -
except that it has the same features as the X11 driver, twofinger scrolling,
etc. Shouldn't be too hard to imagine how a weston touchpad driver would
look like then. The basic approach is to fill a struct SynapticsFrontend
with callbacks for log messages, events, etc. and then call a few functions
to set up the driver. Events are handled by calling SynapticsReadInput()
whenever data is available on the fd, the driver will call the
button/motion/scrolll/touch event callbacks in return.
Changing defaults is simply done by changing bits in the SynapticsParamaters
struct.

A few things to note:
- this driver is simply the X driver untangled into an X-specific bit and
  the core driver. So it doesn't have a good API (or even a nice clean one).
  This is particularly visible with the VMask implementation that are just
  the valuator_mask_* calls from the server. I suspect this will mutate into
  a TouchRec implementation that talks ABS_MT_*
- the internal APIs are by no means finalised (and since they are
  driver-internal anyway they don't ever have to be)
- following the git history is largely pointless. 142 commits, most of them
  tiny and just shuffling code around. plus, I just hacked away instead
  of properly grouping commits. just as a warning...
- the X driver still supports the legacy backends, but the core is now
  evdev in, frontend hooks out.

Comments?

Cheers,
  Peter