VCS[SVN, CVS, Perforce]: Implement "Annotate Previous" context menu.

in annotation editor, including base infrastructure in
VCSBaseEditor. Pass signals of VCSBaseEditor to opaque
VCSBaseEditable (IEditor) for convenience.

Task-number: QTCREATORBUG-503

src/plugins/subversion/subversioneditor.cpp

@@ -137,3 +137,13 @@ QString SubversionEditor::fileNameFromDiffSpecification(const QTextBlock &inBloc
     }
     return QString();
 }
+
+QStringList SubversionEditor::annotationPreviousVersions(const QString &v, QString *actionTextFormat) const
+{
+    bool ok;
+    const int revision = v.toInt(&ok);
+    if (!ok || revision < 2)
+        return QStringList();
+    *actionTextFormat = tr("Annotate revision \"%1\"");
+    return QStringList(QString::number(revision - 1));
+}