Continued #12

- Prepared GUI for preview tga download.
- Restyled texture preview
- Outsourced preview and asset css to new file tasks_assets.css
- Grouped all tasks page css files into new folder css/tasks/
- Small Preview files are smaller now (300 px max)
- Created new interface "Asset" which provides access to informations
about a single asset
- Added generic Asset class Parameter to AssetTask class.