Avoid crash on other Linux due to ref counting issues in Directory cache

[jeremypw]

Fix #2148 (hopefully)

• Directory cache now holds a toggle reference to the Directory
• Move Directory creation related methods together at top of file
• Ensure directories temporarily cached during creation are removed from cache if necessary
• Amend test
• Remove unnecessary null check (cache_lookup should never be called with a null parameter); Amend calling objects to comply
• Add additional tests for directory_cache operations

A separate critical warning during Directory destruction is also fixed.

It was noticed that the creation_key property may no longer be required after ef93c74. The creation_key should be the same as the location now as it is always a folder and the location should no longer change during initialisation.

However, it was retained for now in case there is a corner case that needs it.

[bobby285271]

Yeah I can no longer reproduce the crash with this branch 🥳

[jeremypw]

Just checking that all calls to cache_lookup can be guaranteed non-null.

[jeremypw]

Hmm maybe not. Although the property directory of Files.File is defined as non-nullable, such an object could be pointing to e.g. network:/// in which case the parent directory must be null. I'll have to put that null check in again and amend the property instead.

[jeremypw]

I did manage to get a warning about the creation_key differing from the location when connected to a remote public WebDAV server but then I could not reproduce it. The code should in any case be robust in this situation.

[bobby285271]

Fixes the issue