GTK theme should not claim to provide a native MessageDialog yet

It was providing all possible types, but now MessageDialog is a new
native dialog type, and only on Android at the moment.

Task-number: QTBUG-34784
Change-Id: I2fb288c8d5e176ca4dafbbc310de2f29bbcfc000
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>

src/plugins/platformthemes/gtk2/qgtk2theme.cpp

@@ -87,8 +87,16 @@ QVariant QGtk2Theme::themeHint(QPlatformTheme::ThemeHint hint) const
 
 bool QGtk2Theme::usePlatformNativeDialog(DialogType type) const
 {
-    Q_UNUSED(type);
-    return true;
+    switch (type) {
+    case ColorDialog:
+        return true;
+    case FileDialog:
+        return true;
+    case FontDialog:
+        return true;
+    default:
+        return false;
+    }
 }
 
 QPlatformDialogHelper *QGtk2Theme::createPlatformDialogHelper(DialogType type) const