Pass wxFont by const reference to wxStyledTextCtrl::StyleSetFont()

Since wxStyledTextCtrl::StyleSetFont does not change its 'font' parameter, it
should be a constant reference so we can pass in temporary wxFont objects.

Closes https://github.com/wxWidgets/wxWidgets/pull/321

include/wx/stc/stc.h

@@ -4899,7 +4899,7 @@ public:
 
     // Set style size, face, bold, italic, and underline attributes from
     // a wxFont's attributes.
-    void StyleSetFont(int styleNum, wxFont& font);
+    void StyleSetFont(int styleNum, const wxFont& font);