OutputPane: Use setter for priority and fix default visibility

Do not show the buttons of output views with priority < 0 (instead of
only == -1). Reduce the number of buttons that are shown by default to
the essential ones.

Change-Id: I5b44f18537b3033ce9d616f044a8b54b76988783
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/plugins/coreplugin/messageoutputwindow.h

@@ -20,7 +20,6 @@ public:
 
     QWidget *outputWidget(QWidget *parent) override;
 
-    int priorityInStatusBar() const override;
     void clearContents() override;
 
     void append(const QString &text);