CSS: new 'spacer' class for div

In 1.2.x we used to add a table row with a 'spacer' class to separate
rows in a table.

This commit introduces a new class that can be used in form container
divs to separate field container divs:

Before:
<table>
<tr>...</tr>
<tr class="spacer"></tr>
<tr>...</tr>
</table>
Now:
<div class="form-container">
<div class="field-container">...</div>
<div class="field-container spacer">...</div>
</div>