Avoid checking updates when desktop is stopped

No need to run all the update machinery when there is no client
connected.

This commit also cleans up the stop handling a bit by moving it to
its own method.
diff --git a/common/rfb/VNCServerST.h b/common/rfb/VNCServerST.h
index e00a1f7..584c48c 100644
--- a/common/rfb/VNCServerST.h
+++ b/common/rfb/VNCServerST.h
@@ -195,6 +195,7 @@
     // - Internal methods
 
     void startDesktop();
+    void stopDesktop();
 
     static LogWriter connectionsLog;
     Blacklist blacklist;