Undo most of the changes of r1036645. In stead of calling writeFile()

directly in retrieveItems we now use the new scheduleCustomTask() in
ResourceBase.

There are however a couple of assumptions made:
* The new Task is placed after ChangeRecorderTasks (This is also
  implemented in this commit).
* Duplicate tasks are filtered out. Important because otherwise on
  each change the file (local or remotely) would get written.

svn path=/branches/work/akonadi-ports/kdepimlibs/; revision=1036785