SF Features#116: Add an Acquire From URL option.

SF Features#116: Add an Acquire From URL option.

Add a dialog to download a file from a specific URL directly into Viking,
using the datasource methods.

By default the downloaded file will be opened as a GPX,
 but any file type supported by GPSBabel can be specified
 (as used by the Import from File option).

If the URL does not resolve, does not return a file, or the file can not be interpreted,
 then simply this will give 'Acquisition Error'