Improve path handling in EditorQuickOpenDialog

[KoBeWi]

• Store path history as UIDs
  • Allows "auto-update" paths when files are moved, so your history items won't break
• Support UID paths in the dialog
• When loading history, remove files that no longer exist
  • Only happens at first opening. If a file is gone between openings and you try to open wrong file, error dialog appears and the entry is deleted

I also added get_file_uid() method to EditorFileSystem, which is faster than going through ResourceLoader. We should use it from now where applicable.

[akien-mga]

Since both the new Quick Open dialog and generalized UIDs are new features in 4.4, I think we can get this merged for 4.4 too to fine-tune the experience.

That being said feel free to set it for 4.5 if you think this should bake some more before merging.

[akien-mga]

Thanks!