Support syncing macOS Finder file tags

[alexhaas]

As far as I can tell, Nextcloud does not support syncing tags that are added to files in Finder on macOS (https://support.apple.com/en-us/HT202754). Since Dropbox supports syncing these tags, I think it would be extremely beneficial to add this to the Nextcloud feature set.

Steps to reproduce

1. Install Nextcloud on more than one Mac.
2. Add a tag to a synced file.
3. Tag never syncs to other computers.

Expected behaviour

The tags should be synced (like Dropbox).

Actual behaviour

Nothing happens.

Server configuration

Operating system: macOS 10.13.1

Web server: nginx

Database: SQLite

PHP version: 7.1.7

Nextcloud version: 12.0.3

Updated from an older Nextcloud/ownCloud or fresh install: Fresh install

Where did you install Nextcloud from: Docker hub

[MorrisJobke]

Please report this in the client repo: https://github.com/nextcloud/client/issues