Improve save handling for built-in scripts

[KoBeWi]

This PR improves the way built-in scripts saved state/saving is handled, to bring them pretty on par with normal scripts.

Summary:

• unified the way of getting script name. Script editors have get_name() method for that, but built-in scripts had some weird special case that omitted this method (even though the method itself also had a case for built-in scripts lol). I unified that
• with the above, the "saved" status of built-in scripts is now properly tracked. Before they were always internally "unsaved" (and closing a modified built-in script always opened the save dialog) and never displayed their status, now unsaved built-in scripts properly display (*)
• whenever scene is saved in editor, all built-in scripts that belong to this scene will be marked as saved
• when you save a built-in script (e.g. using Ctrl+Alt+S shortcut), its scene will be saved instead of trying to save the resource and opening a "Save As" dialog

[Paulb23]

Looks good.

Would be good to apply the get_name changed to text_editor as well.

Not sure if we should also change the FILE_SAVE_ALL action to take into account built-in scripts.

[Paulb23]

Second thoughts, rather then having a separate method and signal can we not handle it here?

[KoBeWi]

The _res_saved_callback isn't called for built-in resources. I removed this check, but it was useless anyways.

[KoBeWi]

Would be good to apply the get_name changed to text_editor as well.

Are built-in TextFiles even a thing? Also #28607 strikes again.

Not sure if we should also change the FILE_SAVE_ALL action to take into account built-in scripts.

It would be easier if saving wasn't handled in multiple places :/

save_all_scripts is called during save_external_data and built-in script isn't really an external data, so I didn't handle it. But it probably makes sense for Save All option in menu...

[KoBeWi]

So it turns out that saving scene when saving script brings many problems (who would've thought...). I had to move the scene_saved signal.

[akien-mga]

Thanks!

[akien-mga]

Would need a dedicated cherry-pick PR for 3.x.