The previous path backend created a libinput context attached to a single
device. This is insufficient when we need to use cross-device functionality.
One example of this cross-device functionality include disabling a touchpad
while the trackstick is in use (Lenovo T440 and related models).

This patch merely adds the infrastructure to support multiple devices for a
path backend. Follow-up patches add the function calls to add and remove
devices. This is needed by Xorg input drivers that still make use of the
server's device hotplug mechanisms but want to otherwise use libinput.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>