Support population "groups"

[openjck]

This is something that the Hardware Report currently supports for the GPU Model chart. There are three buttons: INTEL, NVIDIA, and AMD. Clicking one of them hides the population lines corresponding to that manufacturer.

It should be possible to specify these groups in a manifest and have them spit out in a format the Ensemble accepts.

Corresponding Ensemble issue:

mozilla/ensemble#23

[openjck]

I'm happy to add this metadata manually using the transposer. That's what we have been doing so far. @SuYoungHong, if you can tell me which populations should be in which groups, I can handle the grouping on my side.