Add map preview support in Atlas

[historic_bruno]

We now have map preview images displayed during match setup. Currently, to add a preview requires manual work on the part of the artist/designer, to create a properly sized image, move it to the correct location, and paste the filename into the map's XML (for scenarios) or JSON (for random maps).

The simplest enhancement to Atlas would be a new field in Atlas' map settings tab. A more advanced feature would be adding a map preview tool into Atlas, which would automatically size and save a preview image (probably the current camera view, like a screenshot) and update the map data. This would have the bonus of making map previews much easier to update.