E1S7: Rework Verlissh sight event and rewrite messages

It doesn't make sense for Mal Keshar to know beforehand that what Kyara
is saying is true unless the dwarves painted a portrait of Yechnagoth or
something.
E1S7: Rework final passage event, fix shroud refresh issue

Made it so the event uses CLEAR_CAVE_SHROUD and forces a shroud refresh
since otherwise shroud won't update until the event and dialog sequence
is over.
E1S7: rewrite glyph prose, including the legend of Merthiaal
E1S7: prose revision/rewrite