Make sure that kconf_update is run before Okular::Settings is created

This defines /okular.upd to be deterministic, and makes it run by Okular::Part::Part(). This shall make it possible to reorganize some configuration entries, to allow some simplifications in the config dialog, as planned in #45 .

https://techbase.kde.org/Development/Tools/Using_kconf_update#Note_on_side_effects explains under which conditions I believe calling KConfig::checkUpdate() to be save.