Fix for system default font (FS#1545)

Dave tried to fix this by using `wxSystemSettings::GetFont(wxSYS_DEFAULT_GUI_FONT)`, as I had suggested, but it didn't work - because that function returns the wrong font size on OS X, for some reason.

The constant `wxNORMAL_FONT`, which is supposed to be identical, works fine, so I have changed it to use that instead.

I have tested on Linux as well as OS X; this has no adverse effects.