Add support for tags (creating, searching)

Native tagging system could be much more useful if tags could be searched and edited from the phone app. It would be great especially for managing photos as currently gallery cannot use tags and the previews in list view are really small. Even more so, one could use spare idle times like riding on a bus to organize all photos (especially with beta app and it's great previews)

It could also be the only way of tagging several files at once (currently not possible).

I'm aware that I posted this idea on the old ownCloud forum already :smile: but it came back to me when I read [the issue about search behavior](https://github.com/nextcloud/android/issues/543)

Unfortunately I cannot help with coding, I just wanted to ask your opinion on the matter..