Respect the DontUseNativeDialog option for QColorDialog

By reimplementing canBeNativeDialog() we ensure it is only shown
natively if the circumstances are correct. The change was modelled on
how QFileDialog implements the same checks.

Change-Id: If58e33d6d0547e95975d7c0453901fa592eb96a0
[ChangeLog][QtWidgets][QColorDialog] Ensured
QColorDialog::DontUseNativeDialog is respected when showing the dialog.
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/widgets/dialogs/qcolordialog.cpp

@@ -1790,6 +1790,21 @@ void QColorDialogPrivate::retranslateStrings()
     cs->retranslateStrings();
 }
 
+bool QColorDialogPrivate::canBeNativeDialog() const
+{
+    Q_Q(const QColorDialog);
+    if (nativeDialogInUse)
+        return true;
+    if (q->testAttribute(Qt::WA_DontShowOnScreen))
+        return false;
+    if (q->options() & QColorDialog::DontUseNativeDialog)
+        return false;
+
+    QLatin1String staticName(QColorDialog::staticMetaObject.className());
+    QLatin1String dynamicName(q->metaObject()->className());
+    return (staticName == dynamicName);
+}
+
 static const Qt::WindowFlags DefaultWindowFlags =
         Qt::Dialog | Qt::WindowTitleHint | Qt::MSWindowsFixedSizeDialogHint
         | Qt::WindowSystemMenuHint | Qt::WindowCloseButtonHint;

src/widgets/dialogs/qcolordialog_p.h

@@ -110,6 +110,8 @@ public:
     bool handleColorPickingMouseButtonRelease(QMouseEvent *e);
     bool handleColorPickingKeyPress(QKeyEvent *e);
 
+    bool canBeNativeDialog() const;
+
     QWellArray *custom;
     QWellArray *standard;