SelectWidget: adding separator lines

[Charlie Veniot]

This works with my Chrome web browser.  I don't know about other web browsers.

```

<$select tiddler='$:/SiteTitle' tooltip='Choose a new site title'>
<option>A Tale of Two Cities</option>
<hr>
<option>A New Kind of Science</option>
<option value disabled>—————————————</option>
<option>The Dice Man</option>
</$select>

```

[Charlie Veniot]

I found a unicode character that gives nicer results:

```

<option disabled>─────────────────────────────</option>

```

[Charlie Veniot]

[Charlie Veniot]

Since I'm finding myself using this separator in many places, it made sense to macrofy the thing:

```

\define selectOptionSeparator() <option disabled>─────────────────────────────</option>

```

Example:

```

<<selectOptionSeparator>>
<option value={{🥇 Acknowledgements!!title}}>🥇 Acknowledgements</option>
<option value={{❤ Support This Project!!title}}>❤ Support This Project</option>

```