Modify L.Illustrate.Textbox#_enableTyping to disable map dragging on mouseover and enable developers to add a custom class to the icon container.
Style text in textboxes to use light-grey text and a dark grey halo by default.
Darken textbox background when empty so that users don't lose track of empty textboxes.
Focus in the textbox automatically when the textbox is added to the map.  Outline the textbox only when it has focus.
Fix test for L.Illustrate.Create.Pointer.