Allow Nested Templatedossiers.

This PR allows nested `Template Dossiers`. In the `document from Template` the template listing includes also nested templates. 

This change should not be a problem for existing installations, because adding `Template Dossiers` is protected by a separate permission, which only the `Manager` Role owns.

Additionally i cleaned up the mess (:sweat:) in the `templatedossier.py`:
* Move the files in to a subdirectory
* Split them up in to separate files
* Rework the tests (using ftw.testbrowser)
* Pep8
* Pyflakes
* Types.xml 
 * Drop no longer existing Mail behavior pout
 * Fixed copy&paste fail.

Solves https://github.com/4teamwork/opengever.ai/issues/27 

@lukasgraf could you take a look? thx