* replace "In queue" to "Pending" in the downloads header to avoid confusions (all languages)
* use 29bits Clusters size to support huge video resolutions (fixes#2291) (WebmWriter.java)
* add missing changes to WebmMuxer.java (i forget select the audio track)
* 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
