Fix relayouting when dock widget is deleted directly

Usually, it's closed. "delete" isn't very well tested.
When it's deleted the container item becomes empty, so needs
to be resized to QSize(0,0) as well.

There's already a codepath for that but was too restrictive.
It now handles that case too.

Fixes #336

tests/tst_docks.cpp

@@ -5764,8 +5764,12 @@ void TestDocks::tst_deleteDockWidget()
 
     delete dw1;
 
+    // Frame is deleted delayed
+    QTest::qWait(1000);
+
+    m1->multiSplitter()->dumpLayout();
+
     // Dock3 now occupies everything, separator was deleted
-    QEXPECT_FAIL("", "To be fixed", Continue);
     QVERIFY(m1->multiSplitter()->separators().isEmpty());
 }