[wayland] Emit signal before x11 connection gets destroyed

Used in Compositor to destroy the cm_selection. This would crash
otherwise as it calls into xcb after Xwayland is destroyed.

autotests/wayland/kwin_wayland_test.cpp

@@ -62,6 +62,7 @@ WaylandTestApplication::~WaylandTestApplication()
     if (x11Connection()) {
         Xcb::setInputFocus(XCB_INPUT_FOCUS_POINTER_ROOT);
         destroyAtoms();
+        emit x11ConnectionAboutToBeDestroyed();
         xcb_disconnect(x11Connection());
         setX11Connection(nullptr);
     }

composite.cpp

@@ -128,6 +128,12 @@ Compositor::Compositor(QObject* workspace)
                 }
             }, Qt::QueuedConnection
         );
+        connect(kwinApp(), &Application::x11ConnectionAboutToBeDestroyed, this,
+            [this] {
+                delete cm_selection;
+                cm_selection = nullptr;
+            }
+        );
     } else {
         // delay the call to setup by one event cycle
         // The ctor of this class is invoked from the Workspace ctor, that means before

main.h

@@ -152,6 +152,7 @@ public:
 
 Q_SIGNALS:
     void x11ConnectionChanged();
+    void x11ConnectionAboutToBeDestroyed();
     void workspaceCreated();
     void screensCreated();
     void virtualTerminalCreated();

main_wayland.cpp

@@ -86,6 +86,7 @@ ApplicationWayland::~ApplicationWayland()
     if (x11Connection()) {
         Xcb::setInputFocus(XCB_INPUT_FOCUS_POINTER_ROOT);
         destroyAtoms();
+        emit x11ConnectionAboutToBeDestroyed();
         xcb_disconnect(x11Connection());
         setX11Connection(nullptr);
     }