Fix merge edit/delete problem with Custom properties

* Add test for deleted custom list item dangler
* Add test for custom property problem
* Split off GrammaticalSenseFixer to a separate file
* Add CustomPropertyFixer
* Collect all Custom field names
* Remove Custom elements from any rt element if not
  found in list of defined Custom fields

Change-Id: I04d3d077d78687d3ea3cc986b7f4a9bef73b38f0