JBIDE-17818 Visual Editor: Need to handle external html / htm files

@maxandersen @dgolovin @fbricon @alexeykazakov - I need +1 to push it into X branch
It matters. When we handle external file, we need to get model only of html file itself, we cannot get(and don't need) model of js, css, png, etc. which can be imported in html file we open.