Enable client to request image formats other than the currently hardcoded image/png

[prushforth]

Background

In the MapML extension, the map-link URL template is generated with a hard-coded value of "image/png" regardless of whether this format is actually available for the image or tile being requested.

Requirement

For <map-link rel="image">, enable this selection for a layer or layer group in the MapML Settings through a drop-down list of formats that are actually enabled (currently the list is in the capabilities doc). Generate the format=image/___ value based on this selection at the layer or layer group level. For <map-link rel="tile">, where the tiles are being generated by tile-shaped GetMap requests because the "Use Tiles" option is checked but there is no tile caching enabled in the "Tile Caching" tab panel, use the same selection method as for <map-link rel="image"> i.e., the list of formats that are actually enabled per the capabilities document)

For <map-link rel="tile"> where there is in fact a tile cache available, ienable the selection from a list of image formats that are being cached for the layer or layer group, where the list is specified by what image formats are checked in the "Tile Caching" tab for the layer or layer group.