Implement string based search & replace in multiple files.

This uses the same UI as "Rename Symbol".
Moves the actual rename implementation to a static method in
BaseFileFind and uses it for rename symbol and search & replace.
Moves the signal notification for the code model from VCSManager to
the more general FileManager.
Note that as for rename symbol, there's no undo yet.

Task-number: QTCREATORBUG-73