Explicitly set check notification duration to 5s
Add MythBusyNotification type
Add "busy" state support to default themes
Use notifications to display the metadata search in mythvideo

The notification can be dismissed allowing to use the frontend while the search is occurring. Failure to retrieve the data will be shown instead of just being dismissed silently.
While a search is in progress, editing the metadata for that particular video is disabled.

Fixes #11747