[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