CMake: Improve generator selection in kits

Allow to select generator and extragenerator in a nicer way.
Enable support for platforms and toolsets.

Change-Id: I0c9aae635cdca0ec1db7f285e9cb2785172ed338
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/cmakeprojectmanager/cmakekitconfigwidget.h

@@ -98,8 +98,11 @@ public:
     QString toolTip() const override;
 
 private:
+    void changeGenerator();
+
     bool m_ignoreChange = false;
-    QComboBox *m_comboBox;
+    QLabel *m_label;
+    QPushButton *m_changeButton;
     CMakeTool *m_currentTool = nullptr;
 };