backends/libinput: use ID_INPUT_TOUCHPAD to determine touchpad devices

This change introduces a check for the ID_INPUT_TOUCHPAD property to reliably identify touchpad devices. By doing so, we ensure that the logic specific to touchpads is applied only to the relevant devices, improving the overall robustness and accuracy of input handling. this enhancement addresses issues where non-touchpad input devices were incorrectly processed as touchpads, leading to unintended behavior.