Css cleaning

Move some `vbox`/`hbox`/`reverse` classes  from js to less (reflect changes in nbconvert html templates also)

Split `notebook/style.less` into 2 files `app.less` / `style.less`
second one do not contain app-specific css (like menubar, tooltip, celltoolbar).

switch import in master files `style.less` to import from `app.less`.

recompile css.