List available font families in `tkinter`
<p>In many <code>tkinter</code> examples available out there, you may see things like:</p>

<pre><code>canvas.create_text(x, y, font=('Helvetica', 12), text='foo')
</code></pre>

<p>However, this may not work when run in your computer (the result would completely ignore the font parameter). Aparently, the <code>font</code> parameter is ignored if there is any incorrect value in it.</p>

<p>In order to check if the font family is valid, how can I list all available in my system?</p>