LaTeX/Beamer writer: support "blocks" inside columns and other Divs.

Example:

```

<div class="columns">
<div class="column" width="40%">
- Item
</div>
<div class="column" width="60%">
- Item
</div>
</div>
```

Closes #4016.

src/Text/Pandoc/Writers/LaTeX.hs

@@ -371,6 +371,10 @@ toSlides bs = do
   concat `fmap` mapM (elementToBeamer slideLevel) (hierarchicalize bs')
 
 elementToBeamer :: PandocMonad m => Int -> Element -> LW m [Block]
+elementToBeamer _slideLevel (Blk (Div attr bs)) = do
+  -- make sure we support "blocks" inside divs
+  bs' <- concat `fmap` mapM (elementToBeamer 0) (hierarchicalize bs)
+  return [Div attr bs']
 elementToBeamer _slideLevel (Blk b) = return [b]
 elementToBeamer slideLevel  (Sec lvl _num (ident,classes,kvs) tit elts)
   | lvl >  slideLevel = do