Hide dialogs for library clean/scan by default if automatically triggered.

This doesn't show dialogs and progress bars for library clean or scan if
initiated by an add-on or by a JSON-RPC client by default, as they're assumed
to be automatically triggered.

*library.backgroundupdate is still supported to let the user choose to not show
the progress even if user-initiated as it may take a while.
json-rpc: Add "showdialogs" to Audio/VideoLibrary.Clean/Scan. Bump to 6.18.2.