Count of unused tokens in depTree plugin

Very simple - show how many tokens are without a head in the depTree plugin.

This is done by adding to general purpose functions to state - both come with a spec. Rejoice!

It would be helpful to show unused tokens to a user when he hovers this stat - it's however not exactly clear where the best place to put this information is right now. We might want to highlight in text view - so a general state that a token is highlighted (which is different from selected or hovered) might be in order.

We'll see.