Code model: Update on changes from the versioning system.

Add state logic to CppCodeModelManagerInterface, making it aware whether
an indexer is running, protect the update methods from another
invocation while running. Add changed signals to IVersionControl and
VCSManager and wire them to the update methods. Add a menu action for
manually updating.

Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>
Reviewed-by: con <qtc-committer@nokia.com>