Allow stepping through expression execution

[michaelmior]

This consists of two parts:

1. Some way of calculating the order each expression will be executed and the number of steps. (The easiest way is probably to actually execute the expression and record steps.)
2. Controls to step forward and backward through each execution step. This should just change which expression is highlighted using the existing highlighting mechanism.

Note that users should still be able to click on expressions to see how they will be evaluated. In this case, the current step in query execution should be synchronized with whatever expression has been clicked.