downloaded files for offline access

[cpg]

We should have a feature to be able to enable the user to access and use files when the user is totally offline.

Near as we can tell, we can use the Documents/Downloaded folder (see this doc) and later use something from this thread so that files in that folder can be seen from the Files app (and perhaps others, are backed up, etc.)

To access them in our device, we can make a "virtual share" (e.g. Downloaded) where a list of these files that have been downloaded can be seen by the user, opened and also deleted.

[cpg]

Working well, merged in #143 and #139