Unlike impress:stepenter, we emit impress:steprefresh event also
when the "entered" step is the current step. This allows plugins
to reload or redraw objects if needed.
(Note that resize plugin already calls goto() on the active element
for similar purposes when it sees a window resize event. Emitting
impress:steprefresh allows other plugins to join in such a refresh,
and also others can call goto() if a refresh is needed.)
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