KSNI: fix IconThemePath support

IconThemePath is used by a Canonical Patch to enable all Qt apps using
QSystemTrayIcon to transparently provide a SNI with icons exported into
a temporary path. Hence the lack of support breaks icons for all Qt apps
on Ubuntu.

Fix:
- in the dataengine export IconThemePath
- in the systray protocol pick up the value and forward to QML
- in QML set the themePath accordingly on the IconItem
(requires changes in runtime)

https://bugs.launchpad.net/ubuntu/+source/kde-workspace/+bug/1088772


REVIEW: 107702