Fix "New Rule" dropdown issues

This is for https://github.com/adobe/brackets/issues/5611

@jasonsanjose Here's what I did:

1. Multiple stylesheets with same name are distinguished by displaying minimal number of sub-folders next to name.

2. Dropdown closes when main editor is scrolled.

3. If there's not enough room to display dropdown under button, then it's displayed above button. If there's not enough room to the right to display dropdown, then it's moved left so that it's not clipped.

@larz0 Please review how these changes.