web: Add image and css for play and stop buttons

These are based of the patternfly btn-default class, but changed
so that they look different and have an image. They're an appropriate
size.

Added a table cell class for containing these buttons, since we want
less padding around them.

Part of fix for Issue #301
Closes #307

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>