focuspoll: provide a list of events and zero-width events

With this commit focuspoll provides a list of events to callbacks, one (the last) of each followed type. This avoids one event type beeing shadowed by another and fits what we expect from a poller: give the current/last state. The list is ordered, first is older. We also now restore the zero-width of events where a hack was setting an arbitrary one-character width to get real answers from atspi. These changes were needed for plugin highlightcontent from -extra repo.