Add icons for widgets in World Viewer

[mrDIMAS]

UI editor does not have icons for widgets and it is hard to quickly understand to which widget you're looking at. It would be nice to have icons for widgets in here.

To do that, go to editor/src/ui_scene/utils.rs, find fn icon_of(..) { .. } method and add icons there like it is done in editor/src/world/graph/mod.rs icon_of method. All you need to do is to check the actual type of widget and return a respective icon image. This task also involves some searching for icons, you can use any icon with permissive license (CC0, CC BY-SA, etc).