Make ComKoowaTemplateHelperBehavior::isLoaded() smart

[johanjanssens]

Reduce duplicate code. Add a loadScripts() method to KTemplateHelperBehavior that receives a string and handles the checks to see if the scripts have already been loaded or not. Method will return a string, or not.

[johanjanssens]

Suggestion:

static::loadOnce(callable $callback)

static::loadOnce(function() { return '<ktml:script>'; });

Then loadOnce can keep track of passed callables