We provide visual feedback via a toast to the user that, well, they're supposed to wait; but with the benefit of the cache openAddToPlaylistDialog() may return (almost) immediately, which would render the toast otiose (if not a bit confusing). This commit improves that by cancelling the toast once the wait's over
... (by 'abusing' RxJava's ambWith();
ref on compose() and Transformer: https://blog.danlew.net/2015/03/02/dont-break-the-chain/
and for me, first time laying my hands at RxJava so kindly bear with me; open for suggestions)
28109fef38