Canvas MessageBox: make sizeable / newline support

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Hi, i wanted to show a nice welcome message at the C182 and stumbled upon a problem with the MessageBox implementation. It always has a fixed size.

It would be cool if the messageBox could be sized optionally, so I added a small patch introducing an optional parameter.

PR hereby:

If its merged I will ammend the wiki page: https://wiki.flightgear.org/Canvas_MessageBox

# Examples:
      canvas.MessageBox.information("Orignal", "bla bla bla", nil, canvas.MessageBox.Ok);
      canvas.MessageBox.question("Bigger", "BarContent", nil, canvas.MessageBox.Cancel, [300, 300]);