[GREENFOOT-854] No way to select between multiple overlapping objects when calling a method

If you have many objects on top of each other, and you want to pass one as an argument to a method call by clicking on it, there's no way to choose between the objects. Works fine for left-clicking one object, or right-clicking multiple (where we show a context menu to choose).

We should also show a context menu when left-clicking on multiple objects.

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