Move desktop kio slave from kio-extras

As discussed on,
https://mail.kde.org/pipermail/release-team/2015-July/008686.html

desktop:/ kio slave is very much specific to Plasma workspace and
kio-extras will be moved to KDE Applications.

Branched from kio-extras.git, master branch, at commit
17bb1e1bf294c0600b1e351923a880e51ada7278