WIP: highlight repeat block while stepping

Right now, when stepping through code in step mode, we only stop on steps that have animations (which is most of them).

When we have a loop that is something like "While path ahead", there's an animation for checking whether we have a path ahead, so we step and highlighting the while block.

For repeat loops, there's no animation, so when stepping we only stop on/highlight children.  This can be confusing.

This approach attempts to fix that by injecting a statement into BlocklyApps.log for the start of each loop iteration.  Now, when going through our steps we'll stop on/highlight the repeat block.

Downsides:
- Right now I'm just redefining controls_repeat (in maze).  This means if the core controls_repeat were to change for some reason, we would not get that change.
- This is a very targeted change.  I know that there are at least a few other repeat blocks I'll need to address.  Because it's not a general solution, when new blocks are introduced they would also need to know to do this.

Can you think of any better/more generalized ways of doing this?  One thing I was considering was having some sort of injection mechanism - something like Blockly.InjectMeIntoLoops = function () {} that apps can use to customize what happens in the loop.