feature(libinput): Expose function for querying capability

[Johennes]

This extracts the code for querying input device capabilities into a global function so that consumers of the library can use it as well.

An example of how this is used can be found in https://gitlab.com/cherrypicker/unl0kr/-/merge_requests/15.

[kisvegabor]

I agree that it'd be useful. Thanks!