Backport JMarc's fix for bug #6367.

Backport JMarc's fix for bug #6367.

* Undo.cpp:
 - rename cur member of UndoElement to cur_before
 - add new member cur_after, which is set by Undo::endUndoGroup
 - create a new Undo::endUndoGroup variant that takes a Cursor as parameter. We cannot get rid of the old one because it is used for LFUN_COMMAND_SEQUENCE.
* Cursor.cpp:
 - use endUndoGroup(Dociterator const &) for dispatch
 - update Cursor::endUndoGroup to pass cursor.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/branches/BRANCH_2_0_X@40876 a592a061-630c-0410-9148-cb99ea01b6c8