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.
proportional font-size when editing heading cells

@MinRK @takluyver and I had the idea to do this when we were discussing #6225.
Fixed pandoc version detection.
Fix js-tests for new default selector logic.
appropriate heading level when loading from JSON
match font family when editing heading cells
fix more style consistencies
Merge pull request #5 from jdfreder/css-top-default

Fix js-tests for new default selector logic.
avoid unregistering widget model twice

widgets are registered when the comm is created;
closing unregisters them. Calling `close` a second time
should be a no-op.
Merge pull request #6238 from ivanov/cm-heading-fonts

proportional font-size when editing heading cells
Revised pandoc version detection.
Fixed indentation error.
Allow either okay or accept_role to be returned by confirm exit.
Merge pull request #6261 from blink1073/master

Allow either okay or accept_role to be returned by QtConsole confirm exit.
Fixed parse_filename.
fix loginwidget css
Merge pull request #6250 from minrk/double-free-widget

avoid unregistering widget model twice
Merge pull request #6226 from jasongrout/css-top-default

Remove $el_to_style from the widget javascript code; '' now defaults to this.$el
Add EventfulList and EventfulDict trait types.
Address review comments @takluyver