Cleanup tst_Files::fileUploadDownload.

Fix expected_fail by removing wrongly used QSignalSpy.

The test failed few times already because of a timeout. The test do not
measure the backend performance, which seems to be a bit low, so we can
wait 20 sec for a thumbnail.

Fix indentation.
Shared build fix.

The EnginioBackendConnection needs to be exported for autotests.
Use a const static int instead of a macro.
Move EnginioString to a separate file.
Merge pull request #191 from nierob/master

Small fixups