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.

Ideally, we should make a dropUrls(KFileItem) instead of dropUrls(QUrl), so it doesn't have to create a new KFileItem on the fly. But I don't know Dolphin code well enough to make this change.