do not retry downloads if core is shut(ting) down

core/src/main/groovy/com/muwire/core/Core.groovy

@@ -75,7 +75,7 @@ public class Core {
     private final DirectoryWatcher directoryWatcher
     final FileManager fileManager
     
-    private final AtomicBoolean shutdown = new AtomicBoolean()
+    final AtomicBoolean shutdown = new AtomicBoolean()
         
     public Core(MuWireSettings props, File home, String myVersion) {
         this.home = home		

gui/griffon-app/models/com/muwire/gui/MainFrameModel.groovy

@@ -137,6 +137,8 @@ class MainFrameModel {
             core.eventBus.register(FileUnsharedEvent.class, this)
             
             timer.schedule({
+                if (core.shutdown.get())
+                    return
                 int retryInterval = core.muOptions.downloadRetryInterval
                 if (retryInterval > 0) {
                     retryInterval *= 60000