Fix the "Crash on Exit" bug

We were throwing away the Dispatcher before all the keys (namely, leaked
keys) were unloaded. See the comment in plResManager for more details.

Sources/Plasma/NucleusLib/inc/plgDispatch.h

@@ -68,6 +68,8 @@ public:
     virtual void    MsgQueueOnOff(bool) = 0;      // Turn on or off Queued Messages, if off, uses MsgSend Immediately (for plugins)
 
     virtual bool    SetMsgBuffering(bool on) = 0; // On starts deferring msg delivery until buffering is set to off again.
+
+    virtual void    BeginShutdown() = 0;
 };
 
 class plgDispatch