Fix DropArea::numFrames()

Was counting with placeholder items too. This should fix title bars
being wrong in some cases.

src/private/DropArea.cpp

@@ -56,7 +56,7 @@ DropArea::~DropArea()
 
 int DropArea::numFrames() const
 {
-    return m_layout->count();
+    return m_layout->visibleCount();
 }
 
 Frame *DropArea::frameContainingPos(QPoint globalPos) const

tests/tst_docks.cpp

@@ -2548,7 +2548,8 @@ void TestDocks::tst_addAndReadd()
     QVERIFY(fw);
     auto dropArea = m->dropArea();
     dragFloatingWindowTo(fw, dropArea, DropIndicatorOverlayInterface::DropLocation_OutterRight);
-    dock1->frame()->titleBar()->makeWindow();
+    QVERIFY(!dock1->frame()->titleBar()->isVisible());
+    fw->titleBar()->makeWindow();
     m->multiSplitterLayout()->checkSanity();
 
     //Cleanup