Caching scene groups can take a long time while the editor is unresponsive

[KoBeWi]

Tested versions

4.3 after #60965

System information

Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 1060 (NVIDIA; 31.0.15.4633) - Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz (8 Threads)

Issue description

When a project doesn't have scene_groups_cache.cfg, the editor will crawl every scene and fetch node groups. During this process the editor is unresponsive and it may take a looooong time if the scenes are heavy and there is many of them.

• We should add some progress bar for this action, so the user knows that something is happening.
• It would be nice if fetching groups did not involve fully loading the scene, because if it references some heavy resources, they are being repeatedly loaded and unloaded, which is just wasteful. You don't need to do that to know groups.

Steps to reproduce

1. Have a big project
2. Don't have scene_groups_cache.cfg in your .godot folder
3. Open the project
4. ⏳⏳⏳💀

Minimal reproduction project (MRP)

LongGroups.zip
Can be used to test both raised points.

[jsjtxietian]

IMHO should add a Performance label too.

#89739 adddresses the first task, the second one needs more change.