Add legacy panel classes to support old theme selectors

Signed-off-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Add legacy ‘from-top’ class at the view layer, not the model

This blends in to our approach with the other legacy panel classes.
Don’t apply ‘tool-panel’ class to modal panels
Fix workspace model spec now that legacy panel class is added in view
Merge pull request #4305 from atom/ns-add-legacy-panel-classes

Add legacy panel classes for theme support