please review fix for IZ 202681

Please review fix for IZ 202681:
bug: http://netbeans.org/bugzilla/show_bug.cgi?id=202681
fix: http://hg.netbeans.org/cnd-main/rev/01bed0557221

the problem was that sometimes after IDE reopen some files could be 
opened twice (have two opened editors).
To save memory CND uses slightly different getCookie implementation in 
its' DataObjects (see SourceDataObject)
which does not use CookieSet but forward requests to the Lookup.
Because of that it does not cache CppEditorSupport instances and may 
return new instances which breaks editor.
The fix is to have a cache of these in CppEditorSuppertProvider.

The fix is reviewed and verified.

Egor