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/analyzerbase/analyzerplugin.cpp

@@ -66,8 +66,7 @@ bool AnalyzerPlugin::initialize(const QStringList &arguments, QString *errorStri
 
     // Task integration.
     //: Category under which Analyzer tasks are listed in Issues view
-    ProjectExplorer::ProjectExplorerPlugin::taskHub()
-            ->addCategory(Constants::ANALYZERTASK_ID, tr("Analyzer"));
+    ProjectExplorer::TaskHub::addCategory(Constants::ANALYZERTASK_ID, tr("Analyzer"));
 
     return true;
 }