feat: simple custom sound selection

[sickerin]

Added some custom sounds (from pixabay), and you can choose from a few options. Currently, the implementation recycles existing components for the UI, uses radio components. Ideally, we should eventually implement a dropdown for custom sounds, maybe a volume slider, and include the Notification Types, then put it under Notification Settings perhaps.

Just making a PR, in case anyone has any feedback or ideas. This PR addresses some of these issues: https://github.com/zidoro/pomatez/issues/401 https://github.com/zidoro/pomatez/issues/400