cache change detection

[hungl6844]

using the API method getIndexes and the ifModifiedSince parameter, we can compare the return value to a stored list of cached IDs, applying the differences (deleting no-longer used IDs and downloading new ones). The main advantage of this would be to solve the main issue with the cache: any time a song is changed, the cache essentially has to be nuked before amperfy can accurately recognize the change. Because changes to the file change the song ID, tracking changes to the library can be done more efficiently and accurately.

[BLeeEZ]

Thank you for pointing this out. I will take a look at it.