iOS: set StyleHint::StartDragTime to 300

From testing on iOS, StartDragTime should be adjusted
slightly, from 500 (default in qplatformtheme.cpp) down
to 300. This will also affect how long the user needs
to press before the text selection magnifier shows
up for doing text selections.

Change-Id: I42ebfec6f0dc809b5d392412cf8f70d128ee6246
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>

src/plugins/platforms/ios/qiosintegration.mm

@@ -233,6 +233,8 @@ QPlatformServices *QIOSIntegration::services() const
 QVariant QIOSIntegration::styleHint(StyleHint hint) const
 {
     switch (hint) {
+    case StartDragTime:
+        return 300;
     case PasswordMaskDelay:
         // this number is based on timing the native delay
         // since there is no API to get it