Cube bug fix and material color changes

This update fixes a bug related to cube corner changes and allows for material color changes on multiple selected objects.

Changes

• Bug-fix of the cube when changing its round corners.
• Now it is possible to change the material color of multiple selected objects!
• Several improvements in the context menu.
• Exporting features improvements (still not ready for production, though).
• Tutorial improvement, the first step is now to explain how to rotate the camera.