Add alone and watched properties to UserInfo
Add alone property that can be used to determine if user is the only user on device. It uses tristated value on the background so that if it isn't needed, it's not checked. It is also wired up to the file system watcher so that it gets updated if groups change.
Add watched property to UserInfo so that it's not necessary to watch for changes in databases. Reduces some overhead. Note that this doesn't allow disabling wathing. Once it is set to true it remains that way. Separated the watching callback to its own method.
Stop monitoring seat changes if uid is set. Technically it would be possible to create UserInfo object in QML before activation has happened and then set it's uid to another user. In practice that's quite unusual thing to do. Previously it would have replaced the user with current user but now it just stops monitoring for changes instead.
Also check that grp points to something before using it.