Add wxNativeFontInfo::SetSizeOrDefault() and use it in all ports

Reuse the same code for determining the default font size to use if it
wasn't specified in all major ports.

In particular, make wxGTK behaviour compatible with the rest and use the
normal font size in this case instead of the hardcoded value of 12pt.

src/osx/carbon/font.cpp

@@ -301,7 +301,7 @@ void wxFontRefData::Init(float size,
     else
         SetFamily(family);
 
-    SetFractionalPointSize(size < 0 ? wxNORMAL_FONT->GetFractionalPointSize() : size);
+    m_info.SetSizeOrDefault(size);
     SetNumericWeight(weight);
     SetStyle(style);
     SetUnderlined(underlined);