Fix Default Applications KCM window size
Summary:
Config widgets were loading in when a service item was clicked.
This change loads all config widgets at the start so the window can be sized to the largest widget.
BUG: 398799
FIXED-IN: 5.16.0
Reviewers: Plasma, ngraham
Reviewed By: ngraham
Subscribers: ngraham, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D20072