Enhanced draggable bbox area selector

[harry-wood]

The osm.org export tab lets you specify a bbox, and it's nice because it gives you little draggable handles for adjusting the bbox. Do the same here.

[harry-wood]

I've isolated the javascript that does this: http://harrywood.co.uk/maps/examples/openlayers/bbox-selector.html

Some interesting UI questions about dragging a box versus dragging to move. I quite like the way it works in this example now ('drag a new box' button appearing)

So just need to plug this into xapi-ui now

[harry-wood]

Within a week of figuring that out, the OSM export tab now does something even better using leaflet