Simplify updates of "appContentElement" CSS classes

Instead of wipping all the CSS classes in "appContentElement" and adding
them back again along with the new "participants-X" class now only the
"participants-X" and "incall" classes are updated when the number of
participants in the call is updated.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>