[Mac] Unassigned option key sequences do not appear correctly on OSK

**Issue by [mcdurdin](https://github.com/mcdurdin)**
_Mar 28, 2017 at 03:52 GMT_
_Originally opened as https://github.com/sillsdev/keyman/issues/33_

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a couple of observations on Keyman on OSX:

if an option key sequence is undefined in keyboard layout, underlying system keyboard sequence seems to be used.
for the onscreen keyboard, undefined option keys will display the capital letter fo equivalent key in sytem default layout
system (note: this will be different than the actual output as per above dotpoint)

from @andjc