* restore permission request popup previously removed in #2486
* use legacy file picker in cases where saf file picker is not available
* fix missing file check logic in prepareSelectedDownload method (DownloadDialog.java)
* use Java I/O (classic way) on older android versions
* use Storage Access Framework on newer android versions (Android Lollipop or later)
* both changes have the external SD Card write permission
* add option to ask the save path on each download
* warn the user if the save paths are not defined, this only happens on the first NewPipe run (Android Lollipop or later)
* re-work finished mission database
* re-work DownloadMission and bump it Serializable version
* keep the classic Java IO API
* SAF Tree API support on Android Lollipop or higher
* add wrapper for SAF stream opening
* implement Closeable in SharpStream to replace the dispose() method
* do required changes for this API:
** remove any file creation logic from DownloadInitializer
** make PostProcessing Serializable and reduce the number of iterations
** update all strings.xml files
** storage helpers: StoredDirectoryHelper & StoredFileHelper
** best effort to handle any kind of SAF errors/exceptions
Mauricio Colli
Robin
kapodamy
Christian Schabesberger
John Zhen Mo
Coffeemakr