Move GetWeightClosestToNumericValue() to wxFontInfo too

This is similar to a recent commit which moved float to int point size
conversions to wxFontInfo and is done for the same reasons: wxFont and
wxNativeFontInfo can depend on wxFontInfo, but the converse is not true.

No real changes.

src/unix/fontutil.cpp

@@ -826,7 +826,7 @@ void wxNativeFontInfo::SetStyle(wxFontStyle style)
 void wxNativeFontInfo::SetNumericWeight(int weight)
 {
     wxString s;
-    switch ( wxFont::GetWeightClosestToNumericValue(weight) )
+    switch ( wxFontInfo::GetWeightClosestToNumericValue(weight) )
     {
         case wxFONTWEIGHT_THIN:
             s = "thin";