Like open_from_dirs() but scans first the user data dir and then the system data dirs as defined by the XDG_DATA_DIRS environment variable.
Asynchronously looks for a file with base name 'filename' in all the directories defined in 'dirs' and returns a file input stream for it. If the file can not be found this method returns null.
Asynchronously read a stream into memory. This method will close the input stream when done.
