Revert "Rework scene preview thumbnails"

[akien-mga]

Reverts #102313 (and some fixup PRs) as discussed in #102313 (comment).

The feature (better scene previews) is great and wanted, but we need more time to iron out the implementation and have it tested thoroughly. We merged #102313 just before the feature freeze for 4.5 beta, which wasn't a great timing given its scope (that was underestimated).

So we roll it back for 4.5 instead of attempting to pile more fixes in a short timespan, which would put unnecessary pressure on @daniel080400 (who authored the PR) and other maintainers.

Thanks so much to @daniel080400 for the implementation, and the great work on fixing up regressions after merge! This revert shouldn't be seen as a rejection of the feature, just a delay to give ourselves more time to iterate.

We would very much like to see the reverted PR recreated, taking into account the bug reports and suggestions for improvement, so we can continue reviewing and testing, and aim to merge it early in the next dev cycle (4.6) if it's ready.

• The 1st commit is a squash of reverts of fixup PRs, which AFAIU were direct follow-ups to 102313.
  • Fix crash when save_on_focus_loss is enabled #107397 (with an extra change to revert this line made necessary by this commit: https://github.com/godotengine/godot/pull/103972/files#diff-b88f0bde58b731e8dd353707c8011fb7a1e452f0db2c9f15f4b7905d60e5e52eR2139 )
  • Avoid manipulating PackedScene cache when generating scene preview thumbnails #107514
  • Add null check on generating thumbnails for PackedScene #107630
  • Do not remove SubViewport nodes in thumbnail preview scene #107676
• The 2nd commit reverts Rework scene preview thumbnails (reverted) #102313.
• The following issues are fixed:
  • Fixes Window-derived root causes scene thumbnail to get generated every time #107712
  • Fixes Generate Thumbnails takes over half an hour and re-runs at each project launch (Compatibility) #107736
  • Fixes "ERROR: Task 'generate_thumbnails' already exists" when saving a scene. #107747
  • Fixes Godot 4.5 beta 1: Error when saving a scene with a typed dictionary property (presumably a thumbnails problem) #107764
• The following PRs are no longer relevant after this revert, but could be integrated in a remake of the PR:
  • Supersedes/closes Improve EditorPreviewPlugin #107689
  • Supersedes/closes Create dummy thumbnails for scenes that have no visuals #107719
  • Supersedes/closes Fix thumbnail_progress slowdown by batching _iterate() calls. #107744
  • Supersedes/closes Create scene thumbnails on thread when using Compatibility renderer #107757
• The following proposal is no longer implemented:
  • Reopens Improve scene thumbnails godot-proposals#7232

[AThousandShips]

This is a great feature, and amazing job from a first time contributor, unfortunately it didn't get the testing and love it deserved, but we still want it! I will try my best to help when it comes to getting it back!