Add handlers to filesystem view to override actions

Using `$view->setHandler($operation, $handler)` will set the action for
`$operation` in that view to invoke `$handler->$operation($path[, $extra])`.
Passing an invalid handler will result in \OCP\Files\InvalidHandlerException
being thrown. The default view can be targeted via
`\OC\Files\Filesystem::setHandler(...)`.
Use filesystem handlers for files_trashbin

No more copy-then-delete! Just a direct move!