Add QmlJS semantic checker.

* Add SemanticHighlighter to QmlJSTextEditor to update the semantic info
  in a background thread.
* Add QmlJS::Check to run semantic checks on qml and js documents.
* Add a check for incorrect property names.
* Fix hoverhandler to show tool tips from extra selections over help
  tooltips.

src/libs/qmljs/qmljsdocument.h

@@ -41,6 +41,7 @@
 namespace QmlJS {
 
 class Bind;
+class Snapshot;
 
 class QMLJS_EXPORT Document
 {
@@ -96,6 +97,9 @@ private:
     QString _path;
     QString _componentName;
     QString _source;
+
+    // for documentFromSource
+    friend class Snapshot;
 };
 
 class QMLJS_EXPORT Snapshot
@@ -118,6 +122,9 @@ public:
     Document::Ptr document(const QString &fileName) const
     { return _documents.value(fileName); }
 
+    Document::Ptr documentFromSource(const QString &code,
+                                     const QString &fileName) const;
+
     QList<Document::Ptr> importedDocuments(const Document::Ptr &doc, const QString &importPath) const;
     QMap<QString, Document::Ptr> componentsDefinedByImportedDocuments(const Document::Ptr &doc, const QString &importPath) const;
 };