Timeline will check for the property 'editable' on individual items.

  if editable === true
    ignore timeline option 'editable'
    allow item to be edited
  else if editable === false
    ignore timeline option 'editable'
    do not allow item to be edited
  else
    use timeline option 'editable'

If two items are selected and one is not editable, only the editable
item is draggable.

This change required the css class .vis-editable be moved from the
item set to the individual item containers to maintain proper styling.

Overflow simply extends ItemRange with a new getWidth() function that
determines if the content of the range is wider than the range box
itself.

This did require adding a getWidth() function to the Item object that
always just returns this.width, and adjusting Stack.js to use the new
getWidth() function instead of item.width

than the width of the text. This prevents range items with longer text
lines from being completely hidden.
Also update the stacking collision check to handle this case so that the
overflowed text is not hidden by another item.