Proofs should track the current goal

[leotrs]

The statement of a theorem-like should always include a claim. The claim becomes the goal of the corresponding proof. Each step's handrail should include a new button, "show goal", that would display (or highlight, if within viewport) the claim that is the current goal. For this purpose, rsm-make will have to keep track of what is the current goal. Steps can change the goal, and sub-steps generally have a goal that is a claim on their parent step, while top-level steps have a goal that is the goal of the entire proof.

[leotrs]

This is currently implemented with the exception of steps that change the goal.