Support for inserting an item in a CSSList

[FMCorz]

For your consideration.

[raxbg]

I like the idea, but the method name insert suggests that it must have a parameter for position: before or after. Also what if we just map this to the existing replace? Suggestion:

	public function insert($oItem, $oSibling, $sPosition = 'before') {
		$iIndex = array_search($oSibling, $this->aContents);
		if ($iIndex === false) {
			return $this->append($oItem);
		} else if ($sPosition === 'before') {
			$this->replace($oItem, array($oSibling, $oItem));
		} else {
			$this->replace($oItem, array($oItem, $oSibling));
		}
	}

[ziegenberg]

As there was no progress on this for the last five years I created #545.

[ziegenberg]

@oliverklee I think this can now be closed.

[oliverklee]

Indeed it can. Thanks for implementing/finishing this, @ziegenberg! ❤️

Implemented in #545.