Core: Detach status bar handling from global object pool

Instead of relying on addObject notification, provide a
StatusBarManager::addStatusBarWidget() method to create,
register and keep track of an IContext object.

That's essentially what was StatusBarWidget before, but does
not need to be known on the user side. For removal, this
provides a StatusBarManager::destroyStatusBarWidget(),
any not explicitly removed items are handled in response
to ICore::coreAboutToClose()

The StatusBarManager class is fully static now, could
be a namespace.

Change-Id: Ia2bd13b391c3f68c8dfd584b53524a9649cc0787
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/mainwindow.h

@@ -43,7 +43,6 @@ QT_END_NAMESPACE
 
 namespace Core {
 
-class StatusBarWidget;
 class EditorManager;
 class ExternalToolManager;
 class HelpManager;
@@ -66,7 +65,6 @@ class ProgressManagerPrivate;
 class ShortcutSettings;
 class ToolSettings;
 class MimeTypeSettings;
-class StatusBarManager;
 class VersionDialog;
 class WindowSupport;
 class SystemEditor;
@@ -159,14 +157,12 @@ private:
     ProgressManagerPrivate *m_progressManager = nullptr;
     JsExpander *m_jsExpander = nullptr;
     VcsManager *m_vcsManager = nullptr;
-    StatusBarManager *m_statusBarManager = nullptr;
     ModeManager *m_modeManager = nullptr;
     HelpManager *m_helpManager = nullptr;
     FancyTabWidget *m_modeStack = nullptr;
     NavigationWidget *m_leftNavigationWidget = nullptr;
     NavigationWidget *m_rightNavigationWidget = nullptr;
     RightPaneWidget *m_rightPaneWidget = nullptr;
-    StatusBarWidget *m_outputView = nullptr;
     VersionDialog *m_versionDialog = nullptr;
 
     QList<IContext *> m_activeContext;