Fix QML module setup, fixing image provider

[slipher]

Pull Request check-list

Please make sure to review and check all of these items:

• [x] Does the code keep building with this change?
• [x] Do the unit tests pass with this change?
• [x] Is the commit message formatted according to CONTRIBUTING.MD?
• [ ] If this change fixes a bug (or a performance problem), is a regression test (or a benchmark) included?

Description of change

Use the setup recommended for the case of a QML module that needs a plugin class doing custom initialization for image providers at https://www.qt.io/blog/qml-modules-in-qt-6.2. This is a single library (instead of plugin + backing libraries) with the automatically generated plugin class disabled.

What's wrong with the current way? It results in two copies of the module registration, a manually defined one (Q_PLUGIN_METADATA) in the "backing" library and an automatically generated one in the "plugin" library. The linker may let the duplicate symbol definitions slide, but there is yet a consequence in that plugin initialization is not performed if the automatically generated registration code is selected when loading the module. This can be seen in the tests: although there is no test that directly checks whether the image provider is working, it prints several messages like QWARN : tst_controls::IconTests::test_source_should_use_icon_prefix_for_material_icon() qrc:/qt/qml/Fluid/qml/Icon.qml:151:5: QML QQuickImage: Invalid image provider: image://fluidicontheme/action/settings. This commit fixes those messages.

There is no simple way to add a .cpp file to the plugin library instead of the backing library, so that's why the best thing is to request a single instead of split library. Also the library has to be created by qt_add_qml_module instead of add_library, because qt_add_qml_module doesn't handle the case where the library already exists AND the backing + plugin libraries are combined.