Index file dependencies to make moving files faster

[alex-pahdo]

Right now, the FileSystemDock::_find_remaps function searches through all project files for references to update whenever a file is moved or renamed. For large projects, this can mean waiting 5-10 seconds every time. I propose an index file that maps each resource path to a list of all files that depend on it. That way, the _find_remaps function only needs to do a lookup in that index and process a handful of files when a resource is renamed.

Requirements

1. Setting and removing entries in the index when a TRES or TSCN is saved. This could be triggered by a signal.
2. Changing the resource path as a key and any instances as a value in the index, in conjunction with existing changes to the references
3. Regenerating the index if found to be incomplete, similar to reimporting on editor startup
4. Saving and loading the index file to/from Dictionary at appropriate times

I know where to implement most of this, but I put it in Discussions because I am not sure I can ensure coverage of every place in code where the index would need to be updated.

https://github.com/godotengine/godot/blob/a7937fe54cd2c1f407a529e46e2708aea5ce7243/editor/filesystem_dock.cpp#L1190

[alex-pahdo]

A much simpler solution that I just thought of is to add a button to turn that _find_remaps feature on/off so that you can disable it when you know you don't need it.