add a second Delete action, so both Backspace and Delete can be bound to delete....

add a second Delete action, so both Backspace and Delete can be bound to delete.  this is really useful on macbooks which have a backspace but no delete key.  to turn on this behavior, users will have to add the new editor-alternate-delete action to theri bindings, OR  go to Preferences->Mouse/Key and re-load one of the default bindings files

git-svn-id: svn://localhost/ardour2/branches/2.0-ongoing@9232 d708f5d6-7413-0410-9779-e7cbd77b26cf