Make IDocument::fileName a member with setter.

Instead of requiring subclasses to implement a method.
Also renames IDocument::rename to IDocument::setFileName,
since it doesn't really rename any files or such.

Change-Id: I1344025c24d2f74a6a983e04fb0a5245f1f37aad
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/coreplugin/documentmanager.cpp

@@ -385,7 +385,7 @@ void DocumentManager::renamedFile(const QString &from, const QString &to)
     foreach (IDocument *document, documentsToRename) {
         d->m_blockedIDocument = document;
         removeFileInfo(document);
-        document->rename(to);
+        document->setFileName(to);
         addFileInfo(document);
         d->m_blockedIDocument = 0;
     }