Add ability to Delete All Downloads

[jcraigk]

Add ability to clear all downloaded content using a single button. Should appear somewhere on the Downloads page and potentially also in Settings (gear).

[farktronix]

Settings already has a "Delete all downloaded tracks" option. I'm not sure it's worth adding to the download page though, since I don't think deleting everything is a very common thing to do.
Allowing swipe to delete an entire show on the downloaded page might be good though.