Avoid infinite recursion between wxStockGDI and wxSystemSettions

There was a possible recursion from wxStockGDI::GetFont to
wxSystemSettings::GetFont and back, resolve by using direct font
creation, adding fixed system font.

include/wx/osx/font.h

@@ -26,7 +26,8 @@ enum wxOSXSystemFont
     wxOSX_SYSTEM_FONT_MINI,
     wxOSX_SYSTEM_FONT_MINI_BOLD,
     wxOSX_SYSTEM_FONT_LABELS,
-    wxOSX_SYSTEM_FONT_VIEWS
+    wxOSX_SYSTEM_FONT_VIEWS,
+    wxOSX_SYSTEM_FONT_FIXED
 };