Core: Add a static IContext::attach() function

... to simplify the user code.

Some, but not all, potential beneficiaries are adapted.

Change-Id: Ia7d514a0114a4f99f8ee745523435f5b990bd095
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/autotest/testtreeview.h

@@ -5,10 +5,6 @@
 
 #include <utils/navigationtreeview.h>
 
-namespace Core {
-class IContext;
-}
-
 namespace Autotest {
 namespace Internal {
 
@@ -21,9 +17,6 @@ public:
 
     void selectAll() override;
     void deselectAll();
-
-private:
-    Core::IContext *m_context;
 };
 
 } // namespace Internal