[FEATURE REQUEST] Replace libzip with NSFileCoordinator

[michaelstingl]

@felix-schwarz could you elaborate a bit on the pro's and con's?

How is it related to the one?

• [FEATURE REQUEST] Add "Compress/Uncompress" Action #719

[felix-schwarz]

libzip was originally added to implement transparent compression of bundle formats (f.ex. Keynote documents) before uploading.

When using the NSFileCoordinator API for importing files before upload, iOS will now transparently compress bundle documents into a single file that no longer needs special handling by the ownCloud app.

That removes the original need for libzip, so that it can be removed.

If #719 shall still be pursued, however, libzip will continue to be needed. In case that decision should or can't be made now, there's also the option of keeping libzip in the project, but no longer link it into the app until it is needed again.