addRoot method
Adds a Root to the set of roots.
Returns true if root was added, and false if a Root with
the same URI was already present.
Notifies all connected servers of the change to the list of roots, if the
root was added.
Implementation
bool addRoot(Root root) {
final changed = _roots.add(root);
if (changed) _notifyRootsListChanged();
return changed;
}