* 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.
Ideally most of our visitors won't need to make the QML vs JS
distinction. For these cases it'll be handy to be able to access the AST
no matter whether it's a Program or UiProgram.