This adds hotplugging support to the evdev input backend. We use
libudev to monitor changes to input devices in a separate thread.
Removed devices are removed from the devices list, and new devices
are added to the list.
The effect is that controllers are usable immediately after plugging
them without having to manually refresh devices (if they were
configured to be used, of course).
This makes the device ID assigning code common to all backends, by
moving it to AddDevice() instead of copy-pasting or replicating
the logic in the backends.
Also, to prepare for hotplugging, instead of relying on a name usage
count, the new ID assigning system always starts from ID 0 and tries
to assign the first ID that is not used.
Previously, the devices vector would be passed to all backends. They
would then manually push_back to it to add new devices. This was fine
but caused issues when trying to add synchronisation.
Instead, backends now call AddDevice() to fill m_devices so that it is
not accessible from the outside.
Léo Lam