QmlJS semantic highlighter: Abort when file changes.

Fixes the warning in incrementalApplyExtraAdditionalFormats being
triggered when the document is reduced in size while a highlight is
running.

Change-Id: I60e0c8772d511f52ee433d27cc77ba726030b354
Reviewed-on: http://codereview.qt-project.org/4661
Reviewed-by: Leandro T. C. Melo <leandro.melo@nokia.com>

src/plugins/qmljseditor/qmljssemantichighlighter.cpp

@@ -386,6 +386,11 @@ void SemanticHighlighter::rerun(const ScopeChain &scopeChain)
     m_watcher.setFuture(f);
 }
 
+void SemanticHighlighter::cancel()
+{
+    m_watcher.cancel();
+}
+
 void SemanticHighlighter::applyResults(int from, int to)
 {
     if (m_watcher.isCanceled())
@@ -432,3 +437,7 @@ void SemanticHighlighter::updateFontSettings(const TextEditor::FontSettings &fon
     m_formats[LocalStateNameType] = fontSettings.toTextCharFormat(QLatin1String(TextEditor::Constants::C_QML_STATE_NAME));
 }
 
+int SemanticHighlighter::startRevision() const
+{
+    return m_startRevision;
+}