qstandardpaths_ios: add system path for PicturesLocation

For PicturesLocation we add a second path pointing to
system assets. This url is understood by both QFile (using
a dedicated file engine), and QFileDialog, such that if
you point the dialog to the url, it will show a native
image picker dialog.

Change-Id: Ic79393440ab399ed4802cc6d84ec7ad36cbb9369
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>

src/corelib/io/qstandardpaths_ios.mm

@@ -122,8 +122,16 @@ QString QStandardPaths::writableLocation(StandardLocation type)
 QStringList QStandardPaths::standardLocations(StandardLocation type)
 {
     QStringList dirs;
-    const QString localDir = writableLocation(type);
-    dirs.prepend(localDir);
+
+    switch (type) {
+    case PicturesLocation:
+        dirs << writableLocation(PicturesLocation) << QLatin1String("assets-library://");
+        break;
+    default:
+        dirs << writableLocation(type);
+        break;
+    }
+
     return dirs;
 }