title: Xamarin Forms StackLayout: How to set width / height of children to percentages
body: <p>Is there a way to create a vertical stack layout with a button that takes 30% of of the parent, and a text input that takes 70% of the parent? Something like this:</p>
  
  <pre class="lang-xml prettyprint-override"><code>&lt;StackLayout Orientation="Vertical"&gt;
      &lt;Entry Height="70%"&gt;&lt;/Entry&gt;
      &lt;Button Height="30%"&gt;Click me&lt;/Button&gt;
  &lt;/StackLayout&gt;
  </code></pre>
  
  <p>But this doesn't work. Only solution so far is creating a complete Grid item and using that. Are there no other solutions?</p>