Validate slider value, when limits change
The value of slider changes in frontend, and readout, but not in python obj.
Sending data back to python object after slider values change
A comma after `this.touch`
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.
use momentjs for nice dates
appropriate heading level when loading from JSON
match font family when editing heading cells
fix more style consistencies
Creating overall structure and organization.
Work on notebook docs.
Working on notebook docs.
Moving over new interact tutorial.
Pulling content from ipython-in-depth.
Remove placement tutorial,
Code will move into styling tutorial
Removed alignment tutorial,
Code will be moved into styleing tutorial
renamed:    Custom Widgets.ipynb -> Date Picker Widget.ipynb