[KSplashQML] Register DBus before loading the splash theme

Loading the theme and setting everything up can take a while.
Ensure that we catch all setStage calls that might have been emitted before we were fully up.

CCBUG: 405444

CHANGELOG: Reduced likelihood of KSplash not being dismissed properly and only timing out after 30s

Differential Revision: https://phabricator.kde.org/D19753