Fix for property sheet error from not finding labelMorph font

[Chris Double]

I've done a pull request to fix an issue with requesting a property
sheet from a labelMorph on Linux giving an error:

<https://github.com/russellallen/self/pull/17>

This relates to issue 4 but I think that issue is actually a Mac OS X
backend problem. Can anyone confirm if on Mac OS X that the scalableFont
slot doesn't exist? I'm guessing the Mac backend doesn't use x11Globals?

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