New GroupByMenuButton component

[amcclain]

We could use a simple grid-related helper component that binds to a grid model and displays / updates a single-level, full-row groupBy value.

It could accept a list of groupable fields, and could potentially manage hiding of newly grouped and showing of newly ungrouped columns (potentially superceded by #2963 but also maybe a shorter-term option if you also want a convenient control).

UI would be a button with text set to e.g. "By FooValue" and a drop-down menu that shows other available fields. Could also be a select, but I have a hunch that the button might look / feel a bit better.

@jskupsik this would be helpful for our client project, but let's check in together and with Lee before jumping into an implementation.