Refactored and cleaned

- Removed edit parameter where possible from task controller methods
(see #8 ) and jsps/tags
- Removed session scope from task form (form does not get saved on get
anyway)
- Implemented task file download (see #12)
- new texture preview now gets deleted when corresponding asset gets
deleted
- refactored class names from "Facade" to "Form" for gmm.web.forms
- renamed gmm.service.forms to gmm.web.forms
- added and updated javadoc for gmm.web.TaskController
- added default confirmDialog and javascript method confirm(..) for
generic "Really delete?"dialogs
- implemented delete confirmation for task file delete and task delete
- fixed some bugs