Do not emit DecorationSettings::alphaChannelSupportedChanged on tear down

The Compositor is destroyed before the Client and Decorations are
destroyed on shutdown. This meant the Decorations reacted needlessly
on the alpha channel supported. E.g. Aurorae recreated the Decoration
and most likely crashed in Qt.

With this change the signal gets disconnected and the Decorations
just don't do anything.

composite.cpp

@@ -133,6 +133,7 @@ Compositor::Compositor(QObject* workspace)
 
 Compositor::~Compositor()
 {
+    emit aboutToDestroy();
     finish();
     deleteUnusedSupportProperties();
     delete cm_selection;

composite.h

@@ -185,6 +185,7 @@ public Q_SLOTS:
 
 Q_SIGNALS:
     void compositingToggled(bool active);
+    void aboutToDestroy();
 
 protected:
     void timerEvent(QTimerEvent *te);

decorations/settings.cpp

@@ -39,7 +39,7 @@ SettingsImpl::SettingsImpl(KDecoration2::DecorationSettings *parent)
 {
     readSettings();
 
-    connect(Compositor::self(), &Compositor::compositingToggled,
+    auto c = connect(Compositor::self(), &Compositor::compositingToggled,
             parent, &KDecoration2::DecorationSettings::alphaChannelSupportedChanged);
     connect(VirtualDesktopManager::self(), &VirtualDesktopManager::countChanged, this,
         [parent](uint previous, uint current) {
@@ -49,6 +49,12 @@ SettingsImpl::SettingsImpl(KDecoration2::DecorationSettings *parent)
             emit parent->onAllDesktopsAvailableChanged(current > 1);
         }
     );
+    // prevent changes in Decoration due to Compositor being destroyed
+    connect(Compositor::self(), &Compositor::aboutToDestroy, this,
+        [this, c] {
+            disconnect(c);
+        }
+    );
 }
 
 SettingsImpl::~SettingsImpl() = default;