Now only the subscriptions from the selected group by the user will be
loaded.
Also add an option to decide how much time have to pass since the last
refresh before the subscription is deemed as not up to date. This helps
when a subscription appear in multiple groups, since updating in one
will not require to be fetched again in the others.
* full support for Directory API (Android Lollipop or later)
* best effort to handle any kind errors (missing file, revoked permissions, etc) and recover the download
* implemented directory choosing
* fix download database version upgrading
* misc. cleanup
* do not release permission on the old save path (if the user change the download directory) under SAF api
