private boolean isSingleDocRef() {
        return NullSafe.size(explorerNodes) == 1;
    }
Presenter for editing the tags on a single node or adding tags to multiple nodes.
See {@link ExplorerNodeRemoveTagsPresenter} for tag removal on multiple nodes.
stroom-core-client/src/main/java/stroom/explorer/client/presenter/ExplorerNodeEditTagsPresenter.java