Add fade-in, fade-out to Welcome screen and simplify Welcome Screen vs. Loading logic

[abey79]

Things that this issue covers:

• implement a fade-in/fade-out behaviour for the welcome screen
• see if that (and Implement new toned-down style for Welcome Screen #3124) enable relaxing some of the constraint on the loading vs. welcome screen workflow
• in particular, address the web demo workflow break introduced in Open image and mesh files with drag-drop and File->Open #3116, where the welcome screen is no longer displayed after the recording is closed (not sure what that was about, it works now)

[nikolausWest]

Done?

[abey79]

I added the fade-in/out idea to #3244, so we can close this one.