Set the blessed key that keeps pboxd from appearing in the Dock along…

… side our app

If SandboxService.plist doesn't have JoinExistingSession as YES
in the XPCService dictionary, the XPC service will instantiate
its own security session, separate from the app. While this seems
inoccous, it may lead to subtle (or, in our case, not so subtle)
problems. In our case, as a sandboxed NSDocument based app, pboxd
is instantiated on launch. When our app is in a different
security session – as it would be without this key – the Dock
actually shows pboxd running alongside our app, looking like our
app. Effectively, the user sees two copies of our app but one
seems totally broken. Setting JoinExistingSession to YES makes
everyone happy.

SandboxService.plist

@@ -24,6 +24,8 @@
 	<dict>
 		<key>ServiceType</key>
 		<string>Application</string>
+		<key>JoinExistingSession</key>
+		<false/>
 	</dict>
 </dict>
 </plist>