PopupView: Allow overriding of the MD card CSS class
PopupView: Allow overriding of the MD card CSS class

This makes it easy to create a popup using the padding-less md-card-shell, for
example.

jacksonic/foam,osric-the-knight/foam,foam-framework/foam,osric-the-knight/foam,mdittmer/foam,osric-the-knight/foam,foam-framework/foam,jlhughes/foam,jlhughes/foam,mdittmer/foam,foam-framework/foam,jacksonic/foam,jacksonic/foam,jlhughes/foam,osric-the-knight/foam,jlhughes/foam,mdittmer/foam,foam-framework/foam,mdittmer/foam,foam-framework/foam,jacksonic/foam