Fix: can't drop into remote dir

Fix: can't drop into remote dir

KFileItem::isDir() only works when it's created from listDir(),
or from a local QUrl. For a remote QUrl, isDir always returns
false, so we can't use that in supportsDropping().

As a workaround, now supportsDropping() always returns true in
remote dirs -- we don't check if a remote dir is writable when
dropping.