Show progress of library scan in Web UI

[grimsi]

The library scan is a process that can take a very long time and currently the user is not able to see the progress in the Web UI. Ideally the user should be kept updated with a live view of the current stage ("Scanning file system", "Matching games on IGDB", "Downloading images") and maybe even a counter ("Matching games in IGDB: 10/56"). The backend should provide a reactive Flux endpoint which can send realtime updates to the Web UI.