keep only basic changes
keep only basic changes

Kept changes:
- fewer, shorter menu items (too cryptic?), with icons.
- main checkbox does not open menu, but selects all/none.
- total selection count next to main checkbox (unsure if this is to be
kept. It does address #7628).

Removed changes:
- sticky menu.
- checkboxes and counters in menu.

SylvainCorlay/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,cornhundred/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets,jupyter-widgets/ipywidgets,cornhundred/ipywidgets,ipython/ipywidgets,cornhundred/ipywidgets,jupyter-widgets/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets,cornhundred/ipywidgets,SylvainCorlay/ipywidgets,cornhundred/ipywidgets