Assign motivations to an annotation

[cbeer]

Split from #4

A: Motivation menu

I'm proposing we try to follow the menu pattern we're using for the TOC sidebar header menu, but we need to do some additional things.

• I think the goal is an "exposed dropdown menu" as documented on https://material.io/components/menus#exposed-dropdown-menu, with the label ("Motivation") outside of the menu to match the labels in the Target, Style, and Content sections described below.

• As discussed with @mejackreed, we also need to either display one of the motivation options as the default, or show a "Select..." option as the initial option. The latter approach would require us to validate that an actual motivation value has been selecting before saving the new annotation. If we can make the default annotation value configurable, that is probably the best approach.

• The list of options should probably be drawn from the same configured list as used to populate the Motivation filter in the annotation filter option described in Implement basic filtering annotations (by motivation) mirador#2946 (comment)?