Bug 235743 - A deleted file appears in Projects tab

Bug https://netbeans.org/bugzilla/show_bug.cgi?id=235743
IDE lost the last changes (delete file) of project configuration.
After IDE restarting the deleted file appears in the project view as 
broken item.

I do not know why it is a show stopper. It is QA decision.
Bug is random.
Simple workaround: "Project Properties"->"OK" before exit.

Root cause of the bug is a race in two threads:
- first saves configuration
- second updates configuration
So solution is a posting save runnable in the same RP where 
configuration is updated.
It is safe.

Change set:
http://hg.netbeans.org/cnd-main/rev/ac309b0e40b5

Fix was reviewed and verified

Alexander