Wait for transition before loading

[DrRadek]

Brief Description of What This PR Does

Waits for transition to finish before loading (Suggested by hhyyrylainen).
It's currently not possible to implement multi-threaded loading without major slowdown due to Godot 3 limitations (see godotengine/godot#36793).

Related Issues

closes #3566

Progress Checklist

Note: before starting this checklist the PR should be marked as non-draft.

• PR author has checked that this PR works as intended and doesn't
  break existing features:
  https://wiki.revolutionarygamesstudio.com/wiki/Testing_Checklist
  (this is important as to not waste the time of Thrive team
  members reviewing this PR)
• Initial code review passed (this and further items should not be checked by the PR author)
• Functionality is confirmed working by another person (see above checklist link)
• Final code review is passed and code conforms to the
  styleguide.

Before merging all CI jobs should finish on this PR without errors, if
there are automatically detected style issues they should be fixed by
the PR author. Merging must follow our
styleguide.

[hhyyrylainen]

Thanks for the link about the threading thing, certainly very interesting. I did already know that Godot 4.0 makes background resource loading (more) possible.

I edited your PR description to mark this as actually not closing that other issue (which I updated with a new label and comment), but the more recent too long black screen issue #3566

[hhyyrylainen]

Code is fine now and seems to work in game.