Use ::getMarker for fold decorations instead of new DisplayBufferMarker
Add startScreenRow and endScreenRow to DisplayBuffer::findMarkers
Add containedInScreenRange to DisplayBuffer::findMarkers
Add intersection support to DisplayBuffer::findMarkers
Move decoration Editor specs to DisplayBuffer
Decorations can now only be attached to markers.

The basics work. It will render them on the gutter.
Fix decoration specs to work with markers only
Make foldable decorations work again
Only render on updates
Add a spec for screen line changes
Upgrade text-buffer to fix ::findMarkers bug with intersections
Only return decorations for on-screen markers
Add documentation for DisplayBuffer::findMarkers options