Is there a tool to manage dash lenses?
No, there isn't any such Lens manager yet in Ubuntu (24 September, 2012). But some apps are really in their way to appear soon. Such as this lens toogler app. Though this app is in PPA, you can use it.
We hope some more this kind of apps will appear in future and obviously in the standard repository.
Lens Toggler has appeared, it can be installed using
sudo add-apt-repository ppa:aking1012-com/lenstoggle
sudo apt-get update
sudo apt-get install python-lenstoggler`
This is the thing I was asking for: