Improve handling of big media directories (#694)

[Et0h]

As a result of #694 I am have made the following tweaks to how Syncplay handles media directory scanning:

1. Only scan directories and sub-folders in the relevant thread. This means that if you try to load a file via the playlist before this occurs it may fail temporarily, but it does not check if the file of that name is in the media directory (it just does not check sub-folders).
2. It now provides a warning after 2 seconds of not loading, with a further warning every second.
3. Warnings and errors for media directory now state the number of files scanned so far and explicitly states a larger number of files in the folder can also slow things down.