Show a fully-featured MapView, centered on the dropped pin,
which allows panning/zooming. Share button allows opening
in a map application.
Supports showing a description at the top of the screen,
if one is supplied with the event.
Out of scope: showing the local user's location (being
done as a separate story).
Includes some minor tidying: remove duplicate Location,
and make GeoURI parsing a method on that class; fix the
pointer location in MapView (I broke it earlier, whoops!)
