Valgrind: Restructure plugin setup

Bring it more in line with the standard pattern.

Plus some cosmetics.

Change-Id: I2297c4e72892db386c50ece7dc64bdc89f0d9010
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/valgrind/valgrindsettings.h

@@ -187,6 +187,8 @@ class ValgrindGlobalSettings : public ValgrindBaseSettings
 public:
     ValgrindGlobalSettings();
 
+    static ValgrindGlobalSettings *instance();
+
     void toMap(QVariantMap &map) const override;
     void fromMap(const QVariantMap &map) override;