Downloads placenames not currently available in the World Wind file cache or a specified FileStore. The class
derives from Thread and is meant to operate in its own thread.
The sector and resolution associated with the downloader are specified during construction and are final.
Constructs a downloader to retrieve placenames not currently available in a specified file store and places it
The thread returned is not started during construction, the caller must start the thread.
layer - the layer for which to download placenames.
sector - the sector to download data for. This value is final.
resolution - the target resolution, provided in radians of latitude per texel. This value is final.
fileStore - the file store in which to place the downloaded elevations.
listener - an optional retrieval listener. May be null.