Resolve "Modernize splash screen or remove it altogether"

@tbaumgart I removed the splash screen and optimized the startup code to show the UI as early as possible. Note that I also reverted your fix for bug 407902.

I tested with ENABLE_WEBENGINE=ON and it doesn't crash for me, but a) I am testing on macOS b) with a newer version (5.15) and the upstream bug was already fixed in 5.9.4.

I think it's safe to assume this is OK, given that most recent distros ship at least QT 5.12 LTS.

Closes #4 (closed)