Bug 404146

Issue was raised when discussing the workaround in https://phabricator.kde.org/T10464

The relevant code is at:
* https://github.com/KDE/kwin/blob/cfecb1e0770ca6c8fa879124e11b03081342b9ed/tabbox/tabboxhandler.cpp#L281

1. It first checks if a "plasma/look-and-feel/%1/contents/windowswitcher/WindowSwitcher.qml" file exists and uses that.
2. If not, it looks for a "kwin/tabbox/%1/contents/ui/main.qml"
3. Then it uses the hardcoded "informative" QML file "kwin/tabbox/informative/contents/ui/main.qml" as a fallback (I always wondered why it was using informative when I selected certain "look and feel" skins).

So it seems we need a new function perform the QML loading stuff which returns a `QObject` or `nullptr`. Something like this:


	QObject *TabBoxHandlerPrivate::loadSwitcherItem(QString file)
	{
		if (file.isNull()) {
			m_qmlComponent->loadUrl(QUrl::fromLocalFile(file));
			if (m_qmlComponent->isError()) {
				m_qmlComponent.reset(...) // ???
				return nullptr;
			} else {
				QObject *object = m_qmlComponent->create(m_qmlContext.data());
				return object
			}
		}
	}

	QObject *TabBoxHandlerPrivate::createSwitcherItem(bool desktopMode)
	{
		QString lookAndFeelFilepath = desktopMode ? ... : ...
		QString file = lookAndFeelFilepath;
		QObject *object = loadSwitcherItem(file);
		if (object != nullptr) {
			return object;
		}

		QString tabboxFilepath = ...
		QString file = tabboxFilepath;
		QObject *object = loadSwitcherItem(file);
		if (object != nullptr) {
			return object;
		}

		QString informativeTabboxFilepath = ...
		QString file = informativeTabboxFilepath;
		QObject *object = loadSwitcherItem(file);
		if (object != nullptr) {
			QStringList args;
			args << QStringLiteral("The Window Switcher failed to load, using Informative")
			KProcess::startDetached(QStringLiteral("kdialog"), args);

			return object;
		} else {
			QStringList args;
			args << QStringLiteral("The Window Switcher installation is broken, resources are missing.\nContact your distribution about this")
			KProcess::startDetached(QStringLiteral("kdialog"), args);

			return nullptr;
		}
	}