ProjectExplorer: Base KitAspectWidget on BaseAspect

Enables re-using existing functionality.

The "Make mutable" functionality is still oddly placed. I doubt people
find and use it actively.

Change-Id: Ic6aae17e3a7df093f0f5f234f1e6e1afc196a087
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/mcusupport/mcusupportoptions.cpp

@@ -1254,20 +1254,11 @@ class McuDependenciesKitAspectWidget final : public KitAspectWidget
 public:
     McuDependenciesKitAspectWidget(Kit *workingCopy, const KitAspect *ki)
         : KitAspectWidget(workingCopy, ki)
-    {
-        m_emptyWidget = new QWidget();
-    }
+    {}
 
-    ~McuDependenciesKitAspectWidget() override {
-        delete m_emptyWidget;
-    }
     void makeReadOnly() override {}
     void refresh() override {}
-    QWidget *mainWidget() const override {
-        return m_emptyWidget;
-    }
-private:
-    QWidget *m_emptyWidget;
+    void addToLayout(Utils::LayoutBuilder &) override {}
 };
 
 } // Internal