Added reminders to add contact material to manager in ContactMaterial editor

[FilipAlg]

Adds two warnings with corresponding fix it-buttons to the ContactMaterial inspector:

• If the scene does not contain a ContactMaterialManager a warning is displayed and a button is shown which adds a manager to the scene when pressed.
• If the scene's ContactMaterialManager does not contain the currently shown contact material (or the manager does not exist) a warning is displayed and a button is shown which adds the contact material to the manager (disabled if manager does not exist).