Small icon support

Cesium icons will disappear from tool bar and cesium pannel if the EditorPreferenceSettings->UseSmallToolbarIcons is enabled.
We can disable this option and use Cesium4ue as normal, so you can fix this up after you have done those greate works we truely need, like loading landscapes from local data.

This has been addressed in Cesium icons disappear when the "small icons" setting is enabled · Issue #485 · CesiumGS/cesium-unreal · GitHub and will be fixed via Add support for small tool bar icons by javagl · Pull Request #486 · CesiumGS/cesium-unreal · GitHub (probably in the next release).