Flapjack should create a hidden dropdown

[jamesarosen]

• by default, renders the top-level entries (groups and leaves)
• groups should render a "Group Name ►"
• clicking on a group causes the group's children to slide in and replace the current group
• clicking on an entry causes its value to show up in the text <input> (see issue Flapjack should render a text box for searching #1), the corresponding <option> to be selected in the underlying <select>, and the dropdown to close

[jamesarosen]

Initial rendering done in a88c373. Dropdown hidden in 3a21105.