dolphinmainwindow: Reload context menu plugins when configuration changes
Reload the context menu plugins and share the KFileItemActions with the context menu so it stays up-to-date as the user changes the service menu settings.
This uses a KConfigWatcher to trigger settings reload on "kservicemenurc" changes, covering both internal settings changes and external scripts.
Remove the obsolete restart prompt in ContextMenuSettingsPage since changes are now applied instantly.
Edited by Pan Zhang