Remove $el_to_style from the widget javascript code; '' now defaults to this.$el

This seems to make it easiest to select the top-level element.  An alternative is to use special syntax for top-level elements, like in https://github.com/ipython/ipython/pull/6185

This will be followed up by other fixes that let the user set specific common properties of different elements.

This change also adds the .addBack() call, which means that nonempty selectors now apply to this.$el and its descendants, rather than just the descendants of this.$el.
Fix js-tests for new default selector logic.
Merge pull request #5 from jdfreder/css-top-default

Fix js-tests for new default selector logic.
Merge pull request #6226 from jasongrout/css-top-default

Remove $el_to_style from the widget javascript code; '' now defaults to this.$el