[GREENFOOT-833] Add "new X()" above image preview when holding shift and previewing shift-click

As discussed in our meeting, when the user goes to shift-click to add actors into the world, the conceptual link to calling the object's constructor is not clear, especially compared to the case where they right-click the class and select to call the constructor from the context menu. We can help with this by adding text like "new Crab()" above the crab when the user holds shift and moves the mouse over the world, to emphasise that clicking is calling the default constructor of the actor in question.

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Issue metadata

• Issue type: Bug
• Priority: Medium
• Fix versions: 3.7.0