Remove recursive child tasks from `undo.data`.

      <p>Currently the <code>undo.data</code> prevents the sync when an old backup with (daily) recurring tasks is imported because it blows up the nr of lines in <code>undo.data</code>. Short therm solution is to remove the recurrent tasks generated from a few years ago from the <code>undo.data</code> file. To do this one can use the <code>mask++</code> and <code>imask</code> filter as used on <code>backlog.data</code> in <code>Filterbacklog.java</code>.</p>

A better strategy is to only keep for example the latest 14 recurrent tasks in the undo.data this approach is raised in a new issue #88. Before applying a fix to this issue, determine whether issue #88 has not already been fixed instead.

First check issue #88 to determine whether the proposed retention strategy has already been fixed. If not, inspect the undo.data handling and the mask++/imask filtering approach used in Filterbacklog.java. Done means old generated recurrent child tasks no longer make imported backups exceed the undo.data line limit and synchronization succeeds.