VcsManager: Make instance() method return VcsManager * again

This is more convenient as that fixes the code completion for slots
and does allow for calling slots on the singleton without casting.

Change-Id: I1233f449d2b9c9276a29f35d8f8c91c40ec5b352
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/coreplugin/vcsmanager.h

@@ -58,7 +58,7 @@ class CORE_EXPORT VcsManager : public QObject
     Q_OBJECT
 
 public:
-    static QObject *instance();
+    static VcsManager *instance();
 
     static void extensionsInitialized();