By default, leave keyboard-focus with the surface that originally had it
at the end of a dnd operation. This is consistent with the default
behaviour of openbox and mutter.
If the 'focus/followMouse' option is enabled, then focus on the surface
under the cursor at the end of the dnd operation.
Fixes: issue #976
Keyboard focus is not changed during drag, so we need to refocus the
surface under the cursor and the end of a drag-and-drop operation.
Fixes issue #939
