Add remover implementation including views and actions.

* Added `RemoveConditions` which checks preconditions for removing a task (solves #659) 
* Added `Remover` object which handles the removing
* Added `Remove` action to the dossiers `trash` tab
* Added `remove_confirmation` which provides preconditions error messages and confirmation functionality (needs styling implemented in https://github.com/4teamwork/plonetheme.teamraum/pull/271)
 ![bildschirmfoto 2014-11-18 um 18 27 12](https://cloud.githubusercontent.com/assets/485755/5092650/df169c2c-6f50-11e4-8aa0-5a6ff76506c8.png)
 ![bildschirmfoto 2014-11-18 um 18 27 40](https://cloud.githubusercontent.com/assets/485755/5092649/df13b746-6f50-11e4-8641-93a79c518cd0.png)

* Update and added german translations

@lukasgraf please have a look ...
-> `test_document_must_not_have_backreferences`