[Notifications] Delay showing popup until thumbnail is ready

This makes for a much more pleasant visual experience by showing the popup "atomically", instead of showing it, and shortly thereafter resizing it once the thumbnail comes in.

Should the thumbnail take unexpectedly long (> 500ms), the notification will be shown regardless, with a busy indicator, just as before.

Took several screenshots, notification opened with a brief delay but looked a lot cleaner

Full screen screenshots with busy wallpaper can still exceed the 500ms but it's still a massive improvement in visuals.

Hmm, sometimes I'm seeing the notification in task bar as "Plasma", wonder if that is related (accidentally initially opening and then closing and reopening the dialog?), and sometimes it doesn't show at all, sigh :/ might need more work.

Edited by Kai Uwe Broulik

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