Color or filter annotations based on an attribute

[manthey]

If we have a set of annotation elements, it is sometimes useful to have additional properties per element. These could be stored in the element's user property or in the parent annotation's attributes property. If many elements had a numerical value with a common property key or the annotation contained a property in the attributes that was a vector the length of the elements, then this property could be used to color or filter the annotation.

For polygons, we can control the stroke weight, stroke color and opacity, and fill color and opacity; colors could represent multiple or one value; opacity could be used as the filter (but would need to also no longer be hoverable).

For other elements, we may have other parameters we can vary (e.g., use markers instead of circles for points).

[manthey]

Much of this has been added in #279, but leaving this open as there are still needed options.