[3.x] Fix "Changing .import file externally doesn't trigger asset reimport"

[turbohz]

This is an attempt to fix #60730.

I don't use Godot 4 at the moment, but seems easily portable.

The proposed approach is simple: store the md5 hash of the .import file, along with the original and imported file hashes.

When the time comes to check for the need to re-import, check that too.
If we don't find that hash, re-import (which essentially migrates the .md5 file to support the new behavior).

[turbohz]

I found a small issue: if you change the .import file externally, on focusing the editor, the asset will be re-imported, but the Import inspector for the asset won't be updated (if open).

[fire]

To be clear is this sufficient to test?

Steps to reproduce

1. Open icon.png.import
2. Change e.g. repeat to true
3. Switch to editor
4. No reimport
5. Your texture doesn't repeat
6. Your texture doesn't repeat in exported project

[turbohz]

Thanks for looking at this.

To be clear is this sufficient to test?

That's how I tested it, yes (I changed the process/invert_color property instead, because it's easier to see the change).

Additionally, opening an "old" project should cause a full asset re-import, that should happen only once.

[turbohz]

It's worth noting that there's a different proposed fix open for review.

#84974