Investigate allowing users to import a specific style without importing controls first

https://codereview.qt-project.org/c/qt/qtquickcontrols2/+/311816/3#message-0186ccf731d9114b29b8439c086f668be4aa62b0

The theme instantiation should not be done in the plugin, but rather in qtquickcontrolsimpl, so that we can create the theme on demand in such situations.
I'm not sure it was qtquickcontrolsimpl, but somewhere in a shared piece of code available to all styles.

The idea that Ulf is proposing is that

import QtQuick.Controls.Material

should be a supported way of setting the style.