Keep most recent 14 recurrent child tasks in `backlog.data` and `undo.data`.

Currently, if an old backup is imported the nr of lines in the backlog.data and undo.data files are blown up. The current strategy is to delete all recurrent child tasks except for the first. This can be changed to only keep the last 14 or last n tasks. If you fix/apply/solve this, also close issue #87.

It can be fixed by changing the mask++ and iMask filters to find the last entry of the recurrent child task (last entry has all ++++ second to last has ++++-) and count back. For i mask it is matter of finding highest index of that task and counting back. Modify the filters ..removeDupes in FilterBacklog.java.