[kioslave/remote] Fix broken kded module

Summary:
Commit 72e39f083a10 broke the kded module. Turns out that the old code
just forced a refresh of the remote:/ QUrl, so we can just do that
explicitly and remove all the old cruft.

While at it, use a KDirWatcher that just watches the
.local/share/remote/ folder, instead of checking *every* unrelated
QUrl emitted by KDirNotify.

Test Plan:
Adding/deleting/editing .desktop files in .local/share/remoteview now
automatically reloads the remote:/ url in Dolphin or Folder View.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4573