#### Sheet Visibility

Excel enables hiding sheets in the lower tab bar.  The sheet data is stored in
the file but the UI does not readily make it available.  Standard hidden sheets
are revealed in the "Unhide" menu.  Excel also has "very hidden" sheets which
cannot be revealed in the menu.  It is only accessible in the VB Editor!

The visibility setting is stored in the `Hidden` property of sheet props array.

<details>
  <summary><b>More details</b> (click to show)</summary>

| Value | Definition  |
|:-----:|:------------|
|   0   | Visible     |
|   1   | Hidden      |
|   2   | Very Hidden |

With <https://rawgit.com/SheetJS/test_files/HEAD/sheet_visibility.xlsx>:

```js
> wb.Workbook.Sheets.map(function(x) { return [x.name, x.Hidden] })
[ [ 'Visible', 0 ], [ 'Hidden', 1 ], [ 'VeryHidden', 2 ] ]
```

Non-Excel formats do not support the Very Hidden state.  The best way to test
if a sheet is visible is to check if the `Hidden` property is logical truth:

```js
> wb.Workbook.Sheets.map(function(x) { return [x.name, !x.Hidden] })
[ [ 'Visible', true ], [ 'Hidden', false ], [ 'VeryHidden', false ] ]
```
</details>