SessionManager::projects() return a copy instead of a const&

Fixes a crash on session restore for Eike. Iterating over
a const & of the list is dangerous while projects are
added to the list.

Change-Id: I1861b7f1875420769c2493fb0b4012728380efd2
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/projectexplorer/session.cpp

@@ -433,7 +433,7 @@ void SessionManager::closeAllProjects()
     removeProjects(projects());
 }
 
-const QList<Project *> &SessionManager::projects()
+QList<Project *> SessionManager::projects()
 {
     return d->m_projects;
 }