[rfc][icedtea-web][policyeditor] Fix for duplicate codebases when launched from dialog

[Lukasz Dracz]

Hello

> When launching PolicyEditor from a security dialog, the applet's
> codebase may be added to the UI twice. This only affects the list UI -
> each entry shares the same checkbox state, custom permissions, etc., and
> will be written to file as one entry. This happens partially because
> there was some missing validation that the element doesn't already exist
> before it is re-added to the list, and partially because in another
> place where validation was done, the check was performed outside of a
> SwingUtilities.invokeLater block which would later update the UI with
> the new element, without re-checking its presence. This could cause a
> race condition where the element was not yet in the list when it was
> checked but could have already been added by the time it the invokeLater
> ran. This patch fixes these two errors.
> 
> http://icedtea.classpath.org/bugzilla/show_bug.cgi?id=1850

Looks good to me, +0.5! 

Thank you,
Lukasz D