Configuration storage for Wacom tablets and styli

Tablets
-------

Configuration is stored on a per-device model basis, meaning that it's
possible to use the same device, with same configuration on 2 machines
with a shared home directory, or replace a malfunctioning device with the
same model and have the same configuration.

It does not allow [1] having 2 separate tablets of the same model to have
different configurations, whether on a single machine, or using a shared
home directory.

The configuration scheme is:
schema: org.gnome.settings-daemon.peripherals.wacom
path: /org/gnome/settings-daemon/peripherals/wacom/<machine ID>-<device ID>/

where <machine ID> is the D-Bus machine-id for the machine, and
<device ID> is a unique identifier for the tablet model.

[1]: If you're using NFS shared home directories, you can share configurations
between different machines by creating an empty
~/.config/gnome-settings-daemon/no-per-machine-config file.
$ mkdir -p ~/.config/gnome-settings-daemon/
$ touch  ~/.config/gnome-settings-daemon/no-per-machine-config

Note that this needs to happen without any gnome-settings-daemon
running (otherwise half of the configuration would be done in a per-machine
way and half in a generic way), and that previous machine-specific
configurations will be lost.

Stylus
------

Styli use a similar configuration scheme. The identifier for each stylus
is the tool ID, for professional ranges, and a generic identifier for
the consumer ranges that do not support tool ID.

schema: org.gnome.settings-daemon.peripherals.wacom.stylus
or:     org.gnome.settings-daemon.peripherals.wacom.eraser

path: /org/gnome/settings-daemon/peripherals/wacom/<device ID>/<tool ID>/

So each tool can be configured per tablet (so the compatible airbrush stylus
will have different configurations on a Cintiq and an Intuos tablet)

Buttons
-------

schema: org.gnome.settings-daemon.peripherals.wacom.tablet-button
path: /org/gnome/settings-daemon/peripherals/wacom/<device ID>/<button ID>

Testing
-------

You can see all the metadata held about tablets, styli and buttons using
list-wacom, as available in the gnome-settings-daemon sources.

An example output is available at: https://gist.github.com/1688632
