Added live-set-default-font

Updated the previous font submitting code with doc strings and adding ability to set default fonts on systems other than darwin.
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live-set-default-font changes the default font and updates all windows
if emacs is running in XWindows, Win32, or NS drawing systems.

live-set-default-darwin-font calls live-set-default-font only if running
in drawin.

Menlo is only set on drawin.
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Where would you like me to put this in your doc pages? Under the FAQ?