Introduce a timing buffer for the Check for Updates flow to avoid jarring UI.

When a check completes or fails very quickly, the "Checking for Updates" window
flashes briefly before being replaced by the next alert.

This PR introduces a buffering mechanism whereby SPUStandardUserDriver defers
presenting the progress window for 0.3 seconds. If a result is obtained within
that time frame, the progress window is never displayed.

When and if the checking window has actually appeared on screen, a minimum
visible duration of 0.7 seconds must elapse before the panel is dismissed.

Author: danielpunkass