This new feature creates a 'selected windows' buffer which allows
the user to select windows. These windows are then automatically
made the argument for a Wml window operation such as shifting.

This is great for when one wants to apply an action to a set of windows
which are too difficult to describe with Wml expressions.

In addition, I have added a bunch of mouse bindings and key bindings
to this.

This change is still experimental, but it is more intuitive that each
screen has its own history because each screen is generally dedicated to
a specific use case.

I'm going to try this on for size, though it is possible that
per-workspace history mighte prove to be more useful. We'll see.