Core: 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.

Change-Id: I004d6f7fab4375f9902f940f29b4a4f561fc7604
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/actionmanager/actioncontainer.cpp

@@ -398,7 +398,7 @@ TouchBar *ActionContainerPrivate::touchBar() const
     return nullptr;
 }
 
-bool ActionContainerPrivate::canAddAction(Command *action) const
+bool ActionContainerPrivate::canAddAction(Command *action)
 {
     return action && action->action();
 }