Update LabelsMenu

* Remove buttonsUnchecked member and associated methods
* Add new constructor that accepts flag if used in context of opening a
model
* Rename LabelHandler to LabelAddRemoveHandler
* Add new handler: LabelSelectionHandler
Add new member to Constants for default model id

Also change DEFAULT_MODEL to DEFAULT_MODEL_NAME.
Add LabelModelQueryJSO class

Used to store the return from a call to tag/model/query in the API.
Add LabelModelQueryJSOTest

Needs work, though. JSO fixtures are kind of a pain to work with.
Implement tag/model/query when opening a model

This process is used to filter the list of models that are visible to
the user, making selection easier.

This closes #55.