Indicate which tokens are scrubbable

[peterflynn]

For discoverability, there should be some way for the user to see which things are scrubbable.

In decreasing order of risk of being distracting:
a) Faint outline around all values, which gets more salient on rollover.
b) Highlights appear on all values when Ctrl is pressed.
c) Highlights appear on all values when Ctrl is held down for a moment.
d) Highlights appear on all values while scrubbing is already in progress.