Tag selection UI

[rlamorea]

Build a widget for displaying tags in groups, allowing selection/deselection of tags.

• Top level should be "header" groups (unselectable "tags")
• One group of "other" for all parentless tags.
• Subsequent levels should only show direct children of parent.
• If a parent tag is selected and has children, then show children to select.
• If parent tag is unselected, all selected children are automatically unselected as well.
• Widget should be an overlay that expands/contracts as needed.
• Unit tests.