fileselector: Do not call _populate directly in APIs

fileselector: Do not call _populate directly in APIs

Summary:
User application may call some fileselector APIs to configure fileselector instance.
And some of them may call _populate but only last request is needed.
However, T1663 raises that last request might be rejected with expansion option and
multiple requests of _populate.
It's because expansion mode do not allow next _populate until previous one is finished.

So, this patch fixes it via scheduling _populate instead of calling _populate() directly in APIs.

@fix

Maniphest Tasks: T1663