#39 xbindkeys needs to support binding to specific devices

[Nikolai Kondrashov]

To support binding generic frame buttons of specific tablet devices,
xbindkeys needs to support binding actions to specific input devices.

Devices should be identifiable by (a part of the) name at the least. It
would be nice to have matching by vendor, product IDs and serial numbers.
Matching by input device IDs is needed mostly for completeness.

Xbindkeys should bind to *all* the matching devices. Input device list
should be constantly watched for addition/removal of devices to allow
hotplugging.

There needs to be an extension to configuration file syntax which would
allow specifying the originating device for each key combination. The
default would still be matching all devices.

If a configuration file doesn't have bindings to specific devices, then the
device list doesn't need to be watched.

Furthermore, xbindkeys-config should support generating configuration with
bindings to specific devices.