Allow access to system scope setting.

This commit add a Settings::Scope parameter to
Core::ICore::settings(), the default is UserScope,
which falls back to SystemScope. Specifying SystemScope
instead gives exclusive access to the SystemScope.

src/plugins/coreplugin/mainwindow.cpp

@@ -945,6 +945,14 @@ VCSManager *MainWindow::vcsManager() const
     return m_vcsManager;
 }
 
+QSettings *MainWindow::settings(QSettings::Scope scope) const
+{
+    if (scope == QSettings::UserScope)
+        return m_settings;
+    else
+        return m_globalSettings;
+}
+
 EditorManager *MainWindow::editorManager() const
 {
     return m_editorManager;