#32 implemented in serverside, asset class changes

- Moved MessageResponseOperations classes to new package
service.ajax.operations
- Implemented AssetImportOperations fpr #32 serverside
- class AssetTask: renamed newAssetFolder to assetPath. Made Assets more
dynamic (no more absolute paths as object state).
- Asset now know about whos their owner task.
- Refactored gmm.service.tasks. classes.  TaskCreator renamed to
TaskServiceFinder and removed hardcoded calls of service classes to
create asset tasks. For tasks going into the finder, a service will be
found which can handle everything about crating/editing the task.
- Made StringUtil non static and added some methods.
- removed unused path from TaskForm
- made FileNameFilter from FileService static, ignoring cases and
defined static filters for texture and model files in TaskService
subclasses