Reverse share scanning order (Trusted, Buddy, Public) to better handle nested shares

[Tracerneo]

As pointed out by @slook in #3496 (comment)

Currently shares are scanned in order: Public, Buddy, Trusted.
This results in (nested) Trusted/Buddy shares being shared as if they were Buddy/Public shares.
Reversing the scanning order should exclude them from shares of lower trust, as the directories would already be scanned.

Additional suggestion: adding Private (or Exclude, or any other term) scanned before Trusted would create an easy way for creating completely private directories, and exempt them from sharing.

I believe this set needs to be reversed:
https://github.com/nicotine-plus/nicotine-plus/blob/master/pynicotine/shares.py#L303-L309

I would be happy if someone more familiar with the code could look into it, in case the behaviour change would require more code changes.

[slook]

I tried reversing the order of the set, but doing so on its own didn't seem to cause the needed effect, or any effect whatsoever.