Fixes #2608 (Quick Open shows red on first use)

Fixes #2608 (Quick Open shows red on first use) and also makes Quick Open more responsive on large projects by not immediately invalidating the cached file list.

Just eliminating the "red on first use" is a one line change:

```javascript
        var isNoResults = (results.length === 0 && fileList && !this._isValidLineNumberQuery(this.$searchField.val()));
```
But I was also keen to make Quick Open more useful for Brackets again by updating the file list in the background, providing possibly mildly stale results in the meantime.

Ideally, we'd have a spinner while the loading is happening, but I have other things on my plate right now and didn't want to try to build a spinner into the modal bar right now.