OL interaction: have a clear button on the OL geometry controls for story add

[sbenthall]

Phil didn't know how to clear a geometry from the map on the story add control. There should be a 'clear' button that makes this easy.

Should be included on Phil's redesign of story add? See #75

Note: depending on how this is done, I could see an opportunity to contribute to the OL libraries here. Just as the Radio and ToolSwitch controls handle the UI elements and the behavior of a tool switching "panel," analogous classes (maybe 'Button' and 'Action') could be made for this. These would be trivially simple classes, of course, but I think they would be very reusable for cases like this one.

[sbenthall]

Although very minor, general OL appeal