filebrowser 6.x-2.0-rc12

- removed thumbnailing implementations (far too specifics)
- added a generic file handling hook api
- custom thumbnail generation
- curstom metadata
- added a generic way of adding a view (defaults are list-view and icon_view)
- added a generic way of adding a download manager (defaults are public et private, obscured has been removed).
- cleanup of property fixing
#132100 - Corrected custom icons in theme path
#797950 - correction for not hidden extention.