Splash screen does not disappear after startup and covers another popup

[lassoan (Andras Lasso)]

I guess the issue is due to the recent changes caused by extension update checker, which introduced event processing before application startup is completed. I cannot reproduce the problem on Windows, so I cannot verify this, but probably this line is the culprit:

Slicer/Slicer/blob/d1b9611ad098a291f00625674c50913f3ddf9773/Applications/SlicerApp/qSlicerAppMainWindow.cxx#L1218

Instead of relying on timer events processed after startup completed, the application’s startupCompleted() signal should be used instead.

There can be many other places where event processing before application startup may cause problems, so probably it would be better to delay the extension update check (gatherExtensionsHistoryInformationOnStartup): it should be started after application startup is completed.

It is also very odd that extension update check is performed at every startup. If extension update check is successfully completed then it should not be performed again within 10-20 hours.