Disable tool if target easing isn't a cubic bezier

Needed for wat-timeditem-inspector, otherwise I get all these errors when setting the easing to a step function.
I'll need to do a similar thing in wat-step too, but I'll wait until my wat-bezier changes are reviewed.

Also, made some style changes to match wat-timeditem-inspector. (Still not sure why those control point input boxes aren't aligned properly.)