Add filter for WordPress Posts widget content

[stoyan0v]

Changes proposed in this Pull Request:

Add a filter to WordPress Posts widget content, so it will allow to be overwritten.
Issue: #4075

Testing instructions:

• Create a function and hook it to jetpack_display_posts_widget_content
• Modify the $content and return it at the end.
• Example:

add_filter( 'jetpack_display_posts_widget_content', 'change_wordpress_posts_widget_content' );
function change_wordpress_posts_widget_content( $content ) {
	// Change the widget content
	$content = '<h1>Hello world</h1>';
	
	return $content;
}

[eliorivero]

The function esc_html_e outputs instead of returning and will break this.
Replace this with esc_html__.

[stoyan0v]

It has been replaced. Thank you.

[eliorivero]

The initialization of $content to '' is redundant and you can initialize it to $args['before_widget'].

[stoyan0v]

👍

[eliorivero]

Works great, thanks for the contribution!

[thingalon]

Works well, looks good. Merging.