[REVIEW ONLY] Project manager revamp

Note that this pull request is based on jeff/splitview-1x2. I'll make a new PR once that is merged into master.

cc @ingorichter 
Let's see if we need it there first, I think. Most parts of Brackets use `FileSystemEntry` objects which have `isFile` on them. I decided to stick with path strings for this code, because that's pretty much all it needs.