Add a new webtool datasource to allow acquiring data from URLs that return a file.

Adds a built in OpenStreetBugs extension.

Allows defining your own datasource references via a datasources.xml file,
 similar to the external_tools.xml methods.

The URL format reference encodes how the location (and zoom) parameters are substituted.
The file type returned should be specified and is passed on for the -i parameter for GPSBabel.