Aggregation/Utils/ExtensionSystem: Make member functions const/static

readability-make-member-function-const finds lots of member functions
that could be made const. This change just picks getter functions that
really should be const.

readability-convert-member-functions-to-static finds non-static member
functions which do not access this. This change turns most of them
into static ones, but leaves some non static to keep the class API
consistent.

readability-static-accessed-through-instance fixes the places where
the originally non-static, now static functions were called through
instance.

Change-Id: I8cf16c01f7988a7c9d073b5f8ede6a9706b94fb0
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/variablechooser.cpp

@@ -119,7 +119,7 @@ public:
     void updatePositionAndShow(bool);
     void updateFilter(const QString &filterText);
 
-    QWidget *currentWidget();
+    QWidget *currentWidget() const;
 
     int buttonMargin() const;
     void updateButtonGeometry();
@@ -563,7 +563,7 @@ void VariableChooserPrivate::updateFilter(const QString &filterText)
 /*!
  * \internal
  */
-QWidget *VariableChooserPrivate::currentWidget()
+QWidget *VariableChooserPrivate::currentWidget() const
 {
     if (m_lineEdit)
         return m_lineEdit;