TaskHub: Move code to be more conform with other singletons

Make methods static and add a instance() method for Signals/Slots.

Remove ProjectExplorerPlugin::taskHub() method and use the new
ones instead.

Change-Id: Ifae24ff19579fc524cbd61bddc826095c443adfa
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>

src/plugins/projectexplorer/taskwindow.h

@@ -52,7 +52,7 @@ class TaskWindow : public Core::IOutputPane
     Q_OBJECT
 
 public:
-    TaskWindow(ProjectExplorer::TaskHub *taskHub);
+    TaskWindow();
     virtual ~TaskWindow();
 
     void delayedInitialization();