Sync QML module padding to reflect system setting pages

Summary:
In systemsettings 6a9df821ecfaa008206a260b20f595136e254821 it was
determined that the QML KCMs should do their own padding for some
reason.

QML KCMs now do this to match what's expected in system settings.

This change was not mirrored in KMultiDialog (and hence kcmshell5)

Populating d->modules had to move in order to make sure this was
available when the first page was added.

Test Plan:
Ran kcmshell5 autostart (a qwidgets module) main frame still matched
where the buttons are
Ran kcmsheel5 lookandfeel (a qtquick module) main frame now matches
where the buttons are

Reviewers: Plasma

Subscribers: broulik, vpilo, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18739