CppTools: Fix crash on shutdown

We need to wait for the document parser, otherwise the CppModelManager
object might get destroyed first and will be referenced later in
CppEditorSupport::parse().

Change-Id: I1b591640c1da3e63e49ac0b4b8af41aae31b3d43
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>