Allow to group multiple dimensions into one dashboard in the same way that metrics can be grouped

[ncalexan]

#93 added support for grouping multiple metrics into a single Looker dashboard tile. I want to do that, but for multiple dimensions of a single metric. That is, I want to have a single Looker dashboard tile that shows, e.g., total install volume on Windows 7, Windows 8, Windows 10, etc. Placing them on a single tile is both a denser representation and allows easier comparison. Looker supports toggling individual metric lines on and off already. It would be even better if I could show the total of the selected metric lines. (This might avoid adding an "All" dimension for essentially metric in my definitions.)

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