Untitled document support using InaccessibleFileEntry objects.
Merge pull request #4443 from adobe/untitled-documents

Untitled document support
Merge branch 'master' of https://github.com/adobe/brackets into larz/cosmetic-tweaks
Removed add_extensions_sprites as they're no longer needed.
Updated jsTreeSprite.svg so that we can apply rotate animation. Rotate animation applied to disclosure triangles.