VcsBase: Make dependency on CppTools optional

The VcsBaseSubmitEditor uses CppModelManager to collect the symbol names
from the affected files for completion in the commit message.

Move the C++ code model code into CppModelManager, register it in the
plugin manager, and call it via QObject means from the submit editor.

This avoids a hard dependency from VcsBase to CppTools.

Change-Id: I2fb34dbef153c1414820d711e7fc5596bcac1691
Reviewed-by: hjk <hjk@qt.io>

src/plugins/cpptools/cppmodelmanager.h

@@ -230,6 +230,9 @@ public:
 
     void renameIncludes(const QString &oldFileName, const QString &newFileName);
 
+    // for VcsBaseSubmitEditor
+    Q_INVOKABLE QSet<QString> symbolsInFiles(const QSet<Utils::FilePath> &files) const;
+
 signals:
     /// Project data might be locked while this is emitted.
     void aboutToRemoveFiles(const QStringList &files);