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/extensionsystem/pluginview.cpp

@@ -115,8 +115,6 @@ public:
         : m_spec(spec), m_view(view)
     {}
 
-    int columnCount() const { return 4; }
-
     QVariant data(int column, int role) const override
     {
         switch (column) {
@@ -226,8 +224,6 @@ public:
             appendChild(new PluginItem(spec, view));
     }
 
-    int columnCount() const { return 4; }
-
     QVariant data(int column, int role) const override
     {
         if (column == NameColumn) {