Workaround bug in QT_QUICK_CONTROLS_1_STYLE usage in Qt

Summary:
Setting the style manually is necessary as there is only a variable that
applies to both quick controls versions, so it needs to be overwritten to not
cause a warning on every application start. The current way of setting it directly
got defeated by ugly bugs in QQuickControlSettings1, which need to be worked around.
See the added comments for an explanation.

Test Plan:
Set the variables manually, now works consistently everywhere without showing warnings.
For some reason I feel much dirtier now.

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7381