MDL-36002 Drag and Drop: Improvements to keyboard drag and drop

List of fixes:
* Add a simulated drag start event to fix problems with block drag and drop that
  were expecting it.
* Add an access-hidden title for the General section in a course format. This
  is used to provide the text for the drop region (better than the first activity in the section)
* Changed the text in the list to 'Move to "General"' instead of 'Move after General'. This
  makes more sense for activities when you have a list of activities and sections together.