JBIDE-18670 HTML5 Palette: Palette model refactoring

new palette model implemented
Then this method should be commented with TODO and description of planned algorithm, of what is already done, and what is available (insertBeforeId, insertAfterId) for its final implementation.

I can just repeat question that was put to me concerning my model: "Have we to do more coding to the model core every time that we have to add some new groups/items"?.