[wayland] Add support for PlasmaWindowManagement interface

So far this only allows to trigger show desktop functionality and exports
the state.

In future this should be restricted to just one dedicated desktop shell
process.

main_wayland.cpp

@@ -597,6 +597,7 @@ KWIN_EXPORT int kdemain(int argc, char * argv[])
         server->backend()->setInitialWindowSize(initialWindowSize);
     }
 
+    QObject::connect(&a, &KWin::Application::workspaceCreated, server, &KWin::WaylandServer::initWorkspace);
     a.setStartXwayland(parser.isSet(xwaylandOption));
     a.setApplicationsToStart(parser.positionalArguments());
     a.setInputMethodServerToStart(parser.value(inputMethodOption));

wayland_server.cpp

@@ -34,6 +34,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include <KWayland/Server/display.h>
 #include <KWayland/Server/output_interface.h>
 #include <KWayland/Server/plasmashell_interface.h>
+#include <KWayland/Server/plasmawindowmanagement_interface.h>
 #include <KWayland/Server/qtsurfaceextension_interface.h>
 #include <KWayland/Server/seat_interface.h>
 #include <KWayland/Server/shell_interface.h>
@@ -156,6 +157,47 @@ void WaylandServer::init(const QByteArray &socketName)
             }
         }
     );
+    m_windowManagement = m_display->createPlasmaWindowManagement(m_display);
+    m_windowManagement->create();
+    m_windowManagement->setShowingDesktopState(PlasmaWindowManagementInterface::ShowingDesktopState::Disabled);
+    connect(m_windowManagement, &PlasmaWindowManagementInterface::requestChangeShowingDesktop, this,
+        [] (PlasmaWindowManagementInterface::ShowingDesktopState state) {
+            if (!workspace()) {
+                return;
+            }
+            bool set = false;
+            switch (state) {
+            case PlasmaWindowManagementInterface::ShowingDesktopState::Disabled:
+                set = false;
+                break;
+            case PlasmaWindowManagementInterface::ShowingDesktopState::Enabled:
+                set = true;
+                break;
+            default:
+                Q_UNREACHABLE();
+                break;
+            }
+            if (set == workspace()->showingDesktop()) {
+                return;
+            }
+            workspace()->setShowingDesktop(set);
+        }
+    );
+}
+
+void WaylandServer::initWorkspace()
+{
+    if (m_windowManagement) {
+        connect(workspace(), &Workspace::showingDesktopChanged, this,
+            [this] (bool set) {
+                using namespace KWayland::Server;
+                m_windowManagement->setShowingDesktopState(set ?
+                    PlasmaWindowManagementInterface::ShowingDesktopState::Enabled :
+                    PlasmaWindowManagementInterface::ShowingDesktopState::Disabled
+                );
+            }
+        );
+    }
 }
 
 void WaylandServer::initOutputs()

wayland_server.h

@@ -44,6 +44,7 @@ class SeatInterface;
 class SurfaceInterface;
 class OutputInterface;
 class PlasmaShellInterface;
+class PlasmaWindowManagementInterface;
 class QtSurfaceExtensionInterface;
 }
 }
@@ -106,6 +107,7 @@ public:
     int createQtConnection();
     void createInternalConnection();
     void createDummyQtWindow();
+    void initWorkspace();
 
     KWayland::Server::ClientConnection *xWaylandConnection() const {
         return m_xwaylandConnection;
@@ -140,6 +142,7 @@ private:
     KWayland::Server::SeatInterface *m_seat = nullptr;
     KWayland::Server::ShellInterface *m_shell = nullptr;
     KWayland::Server::PlasmaShellInterface *m_plasmaShell = nullptr;
+    KWayland::Server::PlasmaWindowManagementInterface *m_windowManagement = nullptr;
     KWayland::Server::QtSurfaceExtensionInterface *m_qtExtendedSurface = nullptr;
     KWayland::Server::ClientConnection *m_xwaylandConnection = nullptr;
     KWayland::Server::ClientConnection *m_inputMethodServerConnection = nullptr;