Refactor libinput::Device::ConfigData

Rather than an awkward combination of template functions, function
pointers and multiple calls to almost-the-same-but-not-entirely
functions, make ConfigData itself a template and use type erasure to
store them in the config map. This makes the data object aware of its
type and allows us to specialise the reading of config values through
template specialisation. It also removes the need for multiple
constructors and setters in the ConfigData object.