Avoid a crash on Kwin decoration KCM teardown

Summary:
As described in https://bugreports.qt.io/browse/QTBUG-57714  exposing a
QWidget as a context item to QtQuick can crash. Especially as the
engine's context property gets deleted whilst deleting the parent item.

This patch reworks the code so that the models are exposed to QML
directly rather than going through a QWidget.

CCBUG: 373628

Test Plan:
Pressing back whilst in the decoration KCM used to crash every time, now it doesn't.
I still have the buttons.

Reviewers: #plasma, graesslin

Reviewed By: #plasma, graesslin

Subscribers: graesslin, cfeck, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4533

kcmkwin/kwindecoration/kcm.cpp

@@ -100,7 +100,10 @@ ConfigurationModule::ConfigurationModule(QWidget *parent, const QVariantList &ar
     m_quickView->rootContext()->setContextProperty(QStringLiteral("decorationsModel"), m_proxyModel);
     updateColors();
     m_quickView->rootContext()->setContextProperty("_borderSizesIndex", 3); // 3 is normal
-    m_quickView->rootContext()->setContextProperty("configurationModule", this);
+    m_quickView->rootContext()->setContextProperty("leftButtons", m_leftButtons);
+    m_quickView->rootContext()->setContextProperty("rightButtons", m_rightButtons);
+    m_quickView->rootContext()->setContextProperty("availableButtons", m_availableButtons);
+
     m_quickView->rootContext()->setContextProperty("titleFont", QFontDatabase::systemFont(QFontDatabase::TitleFont));
     m_quickView->setResizeMode(QQuickView::SizeRootObjectToView);
     m_quickView->setSource(QUrl::fromLocalFile(QStandardPaths::locate(QStandardPaths::GenericDataLocation, QStringLiteral("kwin/kcm_kwindecoration/main.qml"))));
@@ -406,21 +409,6 @@ void ConfigurationModule::showKNS(const QString &config)
     delete downloadDialog;
 }
 
-QAbstractItemModel *ConfigurationModule::leftButtons() const
-{
-    return m_leftButtons;
-}
-
-QAbstractItemModel *ConfigurationModule::rightButtons() const
-{
-    return m_rightButtons;
-}
-
-QAbstractItemModel *ConfigurationModule::availableButtons() const
-{
-    return m_availableButtons;
-}
-
 bool ConfigurationModule::eventFilter(QObject *watched, QEvent *e)
 {
     if (watched != m_ui) {

kcmkwin/kwindecoration/kcm.h

@@ -47,17 +47,10 @@ public:
 class ConfigurationModule : public KCModule
 {
     Q_OBJECT
-    Q_PROPERTY(QAbstractItemModel* leftButtons READ leftButtons CONSTANT)
-    Q_PROPERTY(QAbstractItemModel* rightButtons READ rightButtons CONSTANT)
-    Q_PROPERTY(QAbstractItemModel* availableButtons READ availableButtons CONSTANT)
 public:
     explicit ConfigurationModule(QWidget *parent = nullptr, const QVariantList &args = QVariantList());
     virtual ~ConfigurationModule();
 
-    QAbstractItemModel *leftButtons() const;
-    QAbstractItemModel *rightButtons() const;
-    QAbstractItemModel *availableButtons() const;
-
     bool eventFilter(QObject *watched, QEvent *e) override;
 
 public Q_SLOTS:

kcmkwin/kwindecoration/qml/Buttons.qml

@@ -63,7 +63,7 @@ Item {
                         id: leftButtonsView
                         anchors.left: parent.left;
                         height: buttonPreviewRow.height
-                        model: configurationModule.leftButtons
+                        model: leftButtons
                         key: "decoButtonLeft"
                     }
                     Item {
@@ -81,7 +81,7 @@ Item {
                         id: rightButtonsView
                         anchors.right: parent.right;
                         height: buttonPreviewRow.height
-                        model: configurationModule.rightButtons
+                        model: rightButtons
                         key: "decoButtonRight"
                     }
                     DropArea {
@@ -144,7 +144,7 @@ Item {
             GridView {
                 id: availableGrid
                 Layout.fillWidth: true
-                model: configurationModule.availableButtons
+                model: availableButtons
                 interactive: false
                 cellWidth: iconLabel.implicitWidth
                 cellHeight: units.iconSizes.small + iCannotBelieveIDoThis.implicitHeight + 4*units.smallSpacing