Braid: Only show samples by X#7

This allows users to subselect which traces display based on their metadata. For example, say you have samples drawn from 3 classes. The metadata attribute is the class, and each sample has one of three values for that class. This feature allows you to only show those samples belonging to a single class, or two classes. There is a placeholder UI component in the HTML for this feature, and nothing yet in the JS. The design will need a little thinking through. Perhaps this feature is always "on" but all classes are selected by default? Or perhaps this feature is always "off" and when you turn it on, no classes are selected by default? Something else? Having this feature interface well with brushes will be important -- potentially some awkward edge cases here (e.g. if someone removes samples, brushes, add samples back that fall in that brush, etc...).