Document Provider: write support test case

having a persistent, writeable relationship between an app and a remote file. This is, what the PR #287 tries to do.

Use cases are:

1. Download a file via SAF

There are many ways to test that. One example:

a) Change your phone background b) Select a file from Seafile via SAF c) it should be downloaded and displayed correctly. :)

(this should already work with version 1.5.1)

2. create a directory via SAF

You can use OpenKeychain for this and the next ones. To use the app, you have to create a public/private keypair once (You don't have to upload it or anything. Just creating one is enough).

a) start openkeychain b) select encrypt files on the left c) select your own mail adress in "encrypt to" (it should auto complete) d) leave "signed by" empty e) add a file via "add file" (local or seafile, doesn't matter) f) select the target file by pressing the "floppy disk" icon on the bottom right. g) enter the seafile root and some repository h) via an icon on the top right, you should be able to create directories. i) these directories should instantly be created on the seafile server.

3. Upload a new file via SAF

a-g) as above h) choose a filename and press the save icon i) gpgkeychain should upload the file j) that file should appear on the seafile server (and be not empty :)

4. Modify an existing file via SAF

basically like 3., just select an already existing file.