Use JSLint-style syntax checking for JavaScript and QML files

JSLint (www.jslint.com) is a tool doing for some basic checks on JavaScript files. We should have a similar feature too for .js files and possibly for .qml file bindings as well. The findings from this jslint processing could be displayed as yellow wavy lines to indicate that something is wrong with that part of the code. The warnings could also appear in the Warnings pane.

I think this kind of static checking would improve the code quality on average, because not everyone starting to use QML is a JavaScript expert.