Make SessionManager::instance() return a SessionManager* again

That's easier to use with Qt5-style connects.

Change-Id: If6afac0718502edc1df6bf0cec1db1466116bbb8
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/plugins/projectexplorer/session.cpp

@@ -157,7 +157,7 @@ SessionManager::~SessionManager()
     delete d;
 }
 
-QObject *SessionManager::instance()
+SessionManager *SessionManager::instance()
 {
    return m_instance;
 }

src/plugins/projectexplorer/session.h

@@ -64,7 +64,7 @@ public:
     explicit SessionManager(QObject *parent = 0);
     ~SessionManager();
 
-    static QObject *instance();
+    static SessionManager *instance();
 
     // higher level session management
     static QString activeSession();