permit to choose image format in wms add window

[landry]

As of now, when adding a wms layer to the map viewer, image/png is used as default. This is fine for vector layers, but when adding raster layers (maps,orthophotos) this gives ugly layers, which are way better rendered using image/jpeg, hence the need for a selector with the available formats from getcapabilities (maybe trimmed to png/jpeg).

One can still hack the wms backend to return jpeg data when asked for format=image/png, but this is rather ugly.

Of course you can object that those layers should be used as baselayers instead... but one never chooses what wms server the end-used will want to use as overlay layers.

[landry]

Oh, and from my understanding of the code, this is hardcoded in the definition of GeoNetwork.tree.WMSTreeGenerator in scripts/map/widgets/tree/WMSTreeGenerator.js

Marking as minor prio while here..