Emit filesystem_changed only once per frame

[Nazarwadim]

This PR makes it so that only one deferred signal is emitted instead of dozens for each updated file. This was done in order to avoid unnecessary FileSystemDock::update_all, which was writing millions of items to the EditorResourcePreview

godot/editor/filesystem_dock.cpp

Lines 338 to 341 in 61accf0

	Array udata;
	udata.push_back(tree_update_id);
	udata.push_back(file_item);
	EditorResourcePreview::get_singleton()->queue_resource_preview(file_metadata, this, "_tree_thumbnail_done", udata);

and creating huge memory usage.

Tested on MRP from #97610.
Before that, it used 9GB of RAM.
Now it use 1GB.
Also, the import time improved 2-3 times.

Closes #97610
Edit: Does not help #95979

[Mickeon]

Doing this may introduce some unexpected regressions for plugins, as this signal is exposed and has been usable outside of internal code. Although this sounds desirable, it needs to be documented at least.

Edit: I'm told this signal is already deferred.

[Saul2022]

Pretty interesting, can’t test rn as i am way too busy, but it could work atleast for ram as my system just has 8gb of ram.

[KoBeWi]

Looks fine. Usually we have a dedicated method for this pattern, e.g. queue_redraw() works like that (i.e. it does a deferred call once per frame). Maybe it's worth applying it for other uses of the filesystem_changed signal? But for now this will suffice.

[Maran23]

This reminds me on a proposal I saw one time. It proposed to have a way to 'batch' signal emits so that everything that connects to that signal is executed just once per frame.
Might be worth evaluating as we have this pattern multiple times in the Engine (e.g. LineEdit). And it would be useful for games too.

[Mickeon]

We may be talking about the same thing. A long time ago there was a proposal (or PR?) for a signal connection flag to defer a single call, regardless of how many times the signal was emitted (behavior similar to queue_redraw). Back in day it was not thought to be useful, but I am very much willing to disagree on that. Just a vague memory.

[KoBeWi]

Related:
godotengine/godot-proposals#3245
https://chat.godotengine.org/channel/core?msg=vLiEN4BEM9M8TziDw

[fire]

Wow great find and the improvements on import time is amazing.

[clayjohn]

Fantastic work! Thank you!