Controlling file selector when using non-native dialogs

When QtQuick.Dialogs is set to use non-native dialogs, the compiler will look for suitable QML files to implement the default appearance of various C++ classes.

This "suitable QML file" is hard defined when using any of the preset QML styles, but when using a custom QML file the file selection is automatic and uncontrollable.

This suggestion is to be able to either be able to define the "suitable QML file" to be used for Dialogs in the custom style, or to be able to do so by default, custom style or not.