ProjectExplorer: Use more const

Clean up action update and use more const variables. Spread some
const over other code, so that const variables can actually be used.

Change-Id: Ia814900ddff294aeafedb2a223d3705990f9b7c1
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/projectexplorer/session.cpp

@@ -574,7 +574,7 @@ QStringList SessionManagerPrivate::dependenciesOrder() const
     return ordered;
 }
 
-QList<Project *> SessionManager::projectOrder(Project *project)
+QList<Project *> SessionManager::projectOrder(const Project *project)
 {
     QList<Project *> result;