Add support for loading fonts from files.

wxFont::AddPrivateFont() can now be used to load a font from a file for the
applications private use. Update the font sample to show this.

Closes #13568.

include/wx/gtk/font.h

@@ -118,4 +118,8 @@ private:
     wxDECLARE_DYNAMIC_CLASS(wxFont);
 };
 
+#ifndef __WXMSW__
+    #define wxHAS_PRIVATE_FONTS 1
+#endif
+
 #endif // _WX_GTK_FONT_H_