Specific input device configuration

Ability to configure specific input devices.

Syntax: something along

input {
    mouse "identifier" {
        // Settings for this device.
    }
}

Resolution for a given device:

1. Search for settings block matching this device's identifier.
2. If not found, use the general block for this device's type.

Identifier: check what other compositors use, guess it's something libinput should be exposing.

For keyboards, it should support per-keyboard XKB config.