create a dedicated FileTag class

At the moment we're inconsistently using QSet<QString> / QList<QString> to hold file tags. This should be unified by using a FileTags container class.

class FileTag:

• should store an id, not a string
• must be constructable from a string
• must be convertable to a string

class FileTags:

• fast to enumerate,
• fast contains and intersection functionality

Looks like FileTags should be a convenience wrapper around std::set<FileTag>.