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/vcsbase/vcsoutputwindow.cpp

@@ -300,7 +300,6 @@ VcsOutputWindow::VcsOutputWindow()
                     TextEditor::TextEditorSettings::behaviorSettings().m_scrollWheelZooming);
     };
 
-    setZoomButtonsEnabled(true);
     updateFontSettings();
     updateBehaviorSettings();