When running on Windows, hide the Engine dialog before calling 'push_state_to_backend()'

When running on Windows, hide the Engine dialog before calling 'push_state_to_backend()'

For some reason we don't understand 'push_state_to_backend()' can interfere with hiding the dialog (causing a partially hidden dialog to remain on screen). It's most likely a timing issue with the Windows version of GTK. Fortunately, reversing the calling order seems to fix it - BUT...

If there's no session loaded yet, the user can be left with a very long wait while nothing seems to be happening. The next thing the user would normally see would be the splash image. So let's display it slightly early (so the user can at least see that something's happening).

Sadly, it's all very kludgy - but a lot better than what we had before...  :)