Make non-native Qt Quick Dialogs styleable like Controls

Currently, Qt Quick Dialogs have dedicated plugins that match the built-in Qt Quick Controls styles. Therefore, non-native dialogs match the look-and-feel of the selected Controls style.

However, this does not work when using a custom Controls style. For example, when we take the Qt Quick Controls - Flat Style example and change the import from "import Qt.labs.platform" to "import QtQuick.Dialogs", we get a dialog that uses the Basic style, rather than the custom Flat style.

It would be good if non-native dialogs could be easily customized/styled too. QTBUG-40568 was closed as Out-of-Scope; let's revisit that decision today.

Use case

A customer is implementing a custom Controls style for branding purposes. They want the same look-and-feel on all platforms - desktop and embedded - so they don't want any native dialogs. However, it is proving rather cumbersome to get the non-native FileDialog to match the custom look-and-feel.