Add specific icon and watermark-icon for subtasks

Used for the icon implementation (see https://github.com/4teamwork/plonetheme.teamraum/pull/259).

Maintask:
![bildschirmfoto 2014-09-29 um 11 26 23](https://cloud.githubusercontent.com/assets/485755/4439644/c18bcece-47ba-11e4-80f1-5770252eb619.png)
Subtask:
![bildschirmfoto 2014-09-29 um 11 26 39](https://cloud.githubusercontent.com/assets/485755/4439643/c1889060-47ba-11e4-89f7-9381153ee748.png)

Close #542
@deiferni please have a look ...