[NONE] Introduce ContentPolicy to handle content operations.

- Added a content policy which contains the logic of constructing content operations (e.g. for deletion of an element).
- Refactored FXDeleteSelectedOnTypePolicy and FXExampleDeleteFirstAnchorageOnClickPolicy to delegate to new transaction policy.
- Removed complex DeleteContentOperation, as logic is now contained within ContentPolicy.
[NONE] Add visibility to interface operations.