Add handler for adding new case to SaveDialogBox
Add "Labels" button to open and save model dialogs

The labels will be used to filter the models, so we can maintain a flat
namespace.
Change color of ActionButtonPanel button on hover

Just like Gmail!
Rename ActionButtonPanel ModelActionPanel

Is this more descriptive? Still verbose.
Add new LabelsMenu popup panel

This is the menu that will be shown when the user clicks on the
"Labels" button in the model open or save dialog.
Modify LabelsMenu to mirror MoreActionsMenu

They should almost share a superclass.
Pop up a LabelsMenu in SaveDialogBox
Pop up a LabelsMenu in OpenDialogBox