Add simple backup solution

Sometimes, it might be handy to backup an entire library of items. For this, the app shall implement the following functionality:

• A user shall be able to navigate to a library and start a backup there.
• This should create a new, temporary Zip archive with all files belonging to the library.
• Finally, the user shall be allowed to save the file (or share it to another app on mobile devices).

In the first iteration, no further functionality is required - however, this would already allow users to manually restore data (e.g. in case a sync with WebDAV/NextCloud/ownCloud goes wrong or whatever). In later versions, we might consider adding a reverse "import" or "restore" function.