Add more references to wxIconBundle to documentation

Add wxIconBundle to the list in the Overview of Available Classes /
Image and bitmap classes as well as to the documentation page for
wxIcon.

Closes https://github.com/wxWidgets/wxWidgets/pull/898

interface/wx/icon.h

@@ -47,7 +47,7 @@
     ::wxNullIcon
 
     @see @ref overview_bitmap, @ref overview_bitmap_supportedformats,
-         wxDC::DrawIcon, wxCursor
+         wxIconBundle, wxDC::DrawIcon, wxCursor
 */
 class wxIcon : public wxGDIObject
 {