Create interface for adding/editing/removing items and events from a timeline

[jeremyboggs]

Create a user interface for adding, editing, and removing items/events from a specific timeline. Specifically:

• Find and select an Item from the Omeka archive to display. Should use the advanced search to narrow down items, and should automatically narrow down items with a valid DublinCore:Date field.
• Add any kind of non-Item event, with Title, Description, StartDate, and EndDate fields.

UI should likely toggle between choosing an existing item, or adding a stand-alone event. Should gracefully degrade if JavaScript is unavailable.

[jeremyboggs]

No longer adding this feature to NeatlineTime.