[FIX] kanban: respect default_order when a groupby is active

The `default_order` attribute of a <kanban> view was applied
for a non-grouped kanban view, but was simply ignored when
the kanban view was grouped, a common situation.
When a groupby is active, the main order is the column being
grouped, but the `default_order` is still useful as
a secondary sort ordering, within the kanban columns.

This patch complements the original patch of
rev. 5c0804ea from PR #520.