Class attributes are managed in a specific way in owl: they are merged
with existing class attributes, and also, they can be defined in
multiple ways (t-att-class, t-attf-class, t-att=['class', ...] and each
of these can be combined together.
Before this commit, the `t-att` syntax was handled as a normal
attribute, and therefore, would not combine as required with existing
classes.
closes#1453
Géry Debongnie
23c7d19ef0