Implement empty slot hiding

[pepamartinec]

Currently, builder leaves the slot markup in the template event if it is empty. But it can sometimes hurt layout and readability of page. For example, when header slot leaves empty <header>.

It would be better, to have the ability to dynamically generate markup only for the non-empty slots.

The markup could be generated automatically, based on the slot name:

{% slot left-panel %}

<div class="left-panel"></div>

Or some settings syntax may be created:

{% slot left-panel wrap : { 'aside', { class : 'my-left-panel' } } %}

<aside class="my-left-panel"></aside>