Prepare KSplash for KDED dropping KSplash support

Summary:
We want to drop workspace code from KDED as it doesn't make much sense
semantically.

This patch ignores the messages from kded, so that they can be dropped
in a future frameworks release and introduces a new signal sent from
startkde.

This puts all workspace related code in workspace.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D1859