Allow tag and icon marker groups to be shown or hidden individually

At the moment there is no way to show or hide tag and icon marker groups individually - it's all or nothing. There should be a facility to do this, similar to the way that topic properties can hidden individually, but from the Map Index. This option should be offered from the context menu when you click on the name of a group in the index.

There would be many uses for this. The most obvious would be when you want to print, publish or show the map with only the key icons or tags, or if you need to show the map to different audiences for whom only some sets of icons or tags are relevant.