Un-virtualize Command and remove an inheritance

This was a left-over from times where we had a QShortcut variant of
Command, which was removed in Qt Creator 3.2.

Change-Id: I62a728f3af352c61bc137461232fc1ed8debccf2
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/plugins/coreplugin/mainwindow.cpp

@@ -71,10 +71,12 @@
 #include <utils/historycompleter.h>
 #include <utils/hostosinfo.h>
 #include <utils/mimeutils.h>
+#include <utils/proxyaction.h>
 #include <utils/qtcassert.h>
 #include <utils/stringutils.h>
 #include <utils/stylehelper.h>
 #include <utils/theme/theme.h>
+#include <utils/touchbar/touchbar.h>
 #include <utils/utilsicons.h>
 
 #include <QActionGroup>