qt6: Port away from QStateMachine

QStateMachine was moved to qtsxml module, which is not ported to
Qt6 yet.

We use QStateMachine for the semantics it gives us, not because of
its implementation. The implementation is trivial, so do it outselves.
We used very little from QStateMachine.

src/private/DragController.cpp

@@ -94,8 +94,49 @@ FallbackMouseGrabber::~FallbackMouseGrabber() {}
 
 }
 
+State::State(MinimalStateMachine *parent)
+    : QObject(parent)
+    , m_machine(parent)
+{
+}
+
+bool State::isCurrentState() const
+{
+    return m_machine->currentState() == this;
+}
+
+MinimalStateMachine::MinimalStateMachine(QObject *parent)
+    : QObject(parent)
+{
+}
+
+template<typename Obj, typename Signal>
+void State::addTransition(Obj *obj, Signal signal, State *dest)
+{
+    connect(obj, signal, this, [this, dest] {
+        if (isCurrentState()) {
+            m_machine->setCurrentState(dest);
+        }
+    });
+}
+
+
+State *MinimalStateMachine::currentState() const
+{
+    return m_currentState;
+}
+
+void MinimalStateMachine::setCurrentState(State *state)
+{
+    if (state != m_currentState) {
+        m_currentState = state;
+        if (state)
+            state->onEntry();
+    }
+}
+
 StateBase::StateBase(DragController *parent)
-    : QState(parent)
+    : State(parent)
     , q(parent)
 {
 }
@@ -112,7 +153,7 @@ StateNone::StateNone(DragController *parent)
 {
 }
 
-void StateNone::onEntry(QEvent *)
+void StateNone::onEntry()
 {
     qCDebug(state) << "StateNone entered";
     q->m_pressPos = QPoint();
@@ -153,7 +194,7 @@ StatePreDrag::StatePreDrag(DragController *parent)
 
 StatePreDrag::~StatePreDrag() = default;
 
-void StatePreDrag::onEntry(QEvent *)
+void StatePreDrag::onEntry()
 {
     qCDebug(state) << "StatePreDrag entered";
     WidgetResizeHandler::s_disableAllHandlers = true; // Disable the resize handler during dragging
@@ -190,7 +231,7 @@ StateDragging::StateDragging(DragController *parent)
 
 StateDragging::~StateDragging() = default;
 
-void StateDragging::onEntry(QEvent *)
+void StateDragging::onEntry()
 {
     if (DockWidgetBase *dw = q->m_draggable->singleDockWidget()) {
         // When we start to drag a floating window which has a single dock widget, we save the position
@@ -331,7 +372,7 @@ StateDraggingWayland::~StateDraggingWayland()
 {
 }
 
-void StateDraggingWayland::onEntry(QEvent *)
+void StateDraggingWayland::onEntry()
 {
     qCDebug(state) << "StateDragging entered";
 
@@ -415,7 +456,8 @@ bool StateDraggingWayland::handleDragMove(QDragMoveEvent *ev, DropArea *dropArea
     return true;
 }
 
-DragController::DragController(QObject *)
+DragController::DragController(QObject *parent)
+    : MinimalStateMachine(parent)
 {
     qCDebug(creation) << "DragController()";
 
@@ -433,8 +475,7 @@ DragController::DragController(QObject *)
     if (usesFallbackMouseGrabber())
         enableFallbackMouseGrabber();
 
-    setInitialState(stateNone);
-    start();
+    setCurrentState(stateNone);
 }
 
 DragController *DragController::instance()
@@ -517,7 +558,7 @@ bool DragController::eventFilter(QObject *o, QEvent *e)
         // On Windows, non-client mouse moves are only sent at the end, so we must fake it:
         qCDebug(mouseevents) << "DragController::eventFilter e=" << e->type() << "; o=" << o;
         activeState()->handleMouseMove(QCursor::pos());
-        return QStateMachine::eventFilter(o, e);
+        return false;
     }
 
     if (isWayland()) {
@@ -546,11 +587,11 @@ bool DragController::eventFilter(QObject *o, QEvent *e)
 
     QMouseEvent *me = mouseEvent(e);
     if (!me)
-        return QStateMachine::eventFilter(o, e);
+        return false;
 
     auto w = qobject_cast<QWidgetOrQuick*>(o);
     if (!w)
-        return QStateMachine::eventFilter(o, e);
+        return false;
 
     qCDebug(mouseevents) << "DragController::eventFilter e=" << e->type() << "; o=" << o
                          << "; m_nonClientDrag=" << m_nonClientDrag;
@@ -563,7 +604,7 @@ bool DragController::eventFilter(QObject *o, QEvent *e)
                 return activeState()->handleMouseButtonPress(draggableForQObject(o), Qt5Qt6Compat::eventGlobalPos(me), me->pos());
             }
         }
-        return QStateMachine::eventFilter(o, e);
+        return false;
     }
     case QEvent::MouseButtonPress:
         // For top-level windows that support native dragging all goes through the NonClient* events.
@@ -585,15 +626,12 @@ bool DragController::eventFilter(QObject *o, QEvent *e)
         break;
     }
 
-    return QStateMachine::eventFilter(o, e);
+    return false;
 }
 
 StateBase *DragController::activeState() const
 {
-    auto set = configuration();
-    if (set.isEmpty())
-        return nullptr;
-    return static_cast<StateBase *>(*(set.begin()));
+    return static_cast<StateBase *>(currentState());
 }
 
 #if defined(Q_OS_WIN)