This adds a `input-default` setting, which is used for input settings if a
device isn't set in `input-devices`. More awkwardly, it also adds an
`input-touchpad` setting, which is used instead of `input-default` for
touchpad devices, so we can separate mouse and touchpad settings even
though they use the same capability and settings.
This no longer sets the input config, and only reads it. If we add a UI
for per-device config, we'll need some IPC mechanism to list connected
devices. (Assuming cosmic-settings can't use libinput directly for that.)
This moves `InputConfig` and `XkbConfig` to a new `cosmic-comp-config`
crate, so they can be used in `cosmic-settings`.
