Core: Fix enabling/disabling of zoomIn/Out buttons in output panes

Amends 48073b2990 to avoid the creation of extra toplevel windows (and
warnings on MS Windows on startup).

Also, ensure that that the initial tab-less application output pane
shows the buttons, albeit in disabled state. All other output panes do
not have to explicitly enable them.

Change-Id: I6c6ef374c09c77e2eb953743071917d9346ca85d
Reviewed-by: hjk <hjk@qt.io>

src/plugins/projectexplorer/appoutputpane.cpp

@@ -282,6 +282,8 @@ AppOutputPane::AppOutputPane() :
 
     connect(SessionManager::instance(), &SessionManager::aboutToUnloadSession,
             this, &AppOutputPane::aboutToUnloadSession);
+
+    setZoomButtonsEnabled(false);
 }
 
 AppOutputPane::~AppOutputPane()