Remove panel classes

Those classes are on the atom-panel

Signed-off-by: Nathan Sobo <nathan@github.com>
Add legacy panel classes to panel in case they don’t exist already

This allows the status bar to be styled correctly with or without the
legacy panel classes being supported in core, which means it can be
merged/released now.
Merge pull request #40 from atom/mb-remove-panel-classes

Remove redundant legacy panel classes from status bar view