* Tweak move item to support moving schematic sheet pin objects.
* Change schematic sheet pin set position to constrain position within
  the parent sheet.
* Schematic sheet pin moving now handled by unified move code.
* Remove old schematic sheet pin move code.
* Add custom client data object for passing hot key information to
  command event handlers.
* Fix a bug that prevented changes to schematic sheet pin objects from
  being undone.