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/coreimpl.h

@@ -68,7 +68,7 @@ public:
     ModeManager *modeManager() const;
     MimeDatabase *mimeDatabase() const;
 
-    QSettings *settings() const;
+    QSettings *settings(QSettings::Scope scope = QSettings::UserScope) const;
     SettingsDatabase *settingsDatabase() const;
     QPrinter *printer() const;