Support for QGuiApplication-based apps

Wraps qApp->style() and creates a QStyle instance of the user's preferred
widget style if qApp->style() returns null. This makes it safe to set
QT_QUICK_CONTROLS_STYLE=org.kde.desktop and still run applications based
on QuickControls2 (QGuiApplication) without crashing.

BUG: 396287
Differential Revision: https://phabricator.kde.org/D14000