Align information set and event terminology

[d-kad]

Issues closed by this PR

Closes #1109

Description of the changes in this PR

One API change. MixedBehaviorProfile.infoset_probs was the only information-set accessor that
also contained the chance player's events; infoset_values and infoset_regrets never did.
It now covers personal players only, and a new event_probs (returning EventProbVector) takes the events.
beliefs covers chance nodes and is normalised by the event's reach probability.
The helper feeding infoset_probs still carried its pre-split name and annotation, looking like a leftover from
before Event existed rather than a deliberate design; happy to revert if needed.

Docstrings, error messages, and internal iterators in behavmixed.pxi are corrected,
and Game.reveal no longer assumes an information set.
In the GUI, the node-properties dialog, the move-properties dialog, the append/insert move popup,
the drag-and-drop popup, and the hover tooltip all say Event at chance nodes.
formats.efg.rst describes chance-node fields in event terms (the file format is unchanged), and
gui.efg.building.rst records the chance-player variants of the dialog labels.