wayland: Add needed API to WindowBeingDragged

So it can act as a substitute for FloatingWindow, which we don't
always have during a drag

src/private/MultiSplitter.cpp

@@ -441,14 +441,12 @@ QRect MultiSplitter::rectForDrop(const WindowBeingDragged *wbd, Location locatio
                                  const Layouting::Item *relativeTo) const
 {
     Layouting::Item item(nullptr);
-    FloatingWindow *fw = wbd ? wbd->floatingWindow() : nullptr;
-    if (!fw)
+    if (!wbd)
         return {};
 
-    Layouting::ItemContainer *root = fw->dropArea()->rootItem();
-    item.setSize(fw->size().boundedTo(root->maxSizeHint()));
-    item.setMinSize(root->minSize());
-    item.setMaxSizeHint(root->maxSizeHint());
+    item.setSize(wbd->size().boundedTo(wbd->maxSize()));
+    item.setMinSize(wbd->minSize());
+    item.setMaxSizeHint(wbd->maxSize());
 
     Layouting::ItemContainer *container = relativeTo ? relativeTo->parentContainer()
                                                      : m_rootItem;

src/private/WindowBeingDragged.cpp

@@ -13,6 +13,7 @@
 #include "DragController_p.h"
 #include "Logging_p.h"
 #include "Utils_p.h"
+#include "DropArea_p.h"
 
 using namespace KDDockWidgets;
 
@@ -102,3 +103,31 @@ QStringList WindowBeingDragged::affinities() const
 {
     return m_affinities;
 }
+
+QSize WindowBeingDragged::size() const
+{
+    if (m_floatingWindow)
+        return m_floatingWindow->size();
+
+    return QSize();
+}
+
+QSize WindowBeingDragged::minSize() const
+{
+    if (m_floatingWindow) {
+        Layouting::ItemContainer *root = m_floatingWindow->dropArea()->rootItem();
+        return root->minSize();
+    }
+
+    return {};
+}
+
+QSize WindowBeingDragged::maxSize() const
+{
+    if (m_floatingWindow) {
+        Layouting::ItemContainer *root = m_floatingWindow->dropArea()->rootItem();
+        return root->maxSizeHint();
+    }
+
+    return {};
+}

src/private/WindowBeingDragged_p.h

@@ -42,6 +42,15 @@ public:
     ///@brief returns the affinities of the window being dragged
     QStringList affinities() const;
 
+    ///@brief size of the window being dragged contents
+    QSize size() const;
+
+    /// @brief returns the min-size of the window being dragged contents
+    QSize minSize() const;
+
+    /// @brief returns the max-size of the window being dragged contents
+    QSize maxSize() const;
+
 private:
     Q_DISABLE_COPY(WindowBeingDragged)
     QPointer<FloatingWindow> m_floatingWindow;