Make the Refresh File Tree command clear caches

This ensures a full refresh.

As suggested in #7929.
Currently, refreshFileTree is asynchronous but it ends at a deterministic point.

I just searched the extension registry... It turns out that the only code that uses the returned promise is [NJ's bower extension](https://github.com/njx/brackets-bower/blob/master/main.js#L110). That code is a bit fishy anyhow because it predates file watchers.

So, it should be safe to change the behavior of refreshFileTree to be a "fire and forget" kind of function, which definitely simplifies things.