Allow dynamic creation of skins

[uramer]

Currently the only way to create a skin is by loading it from an XML node or file. It would be much more convenient and performant to use ResourceSkin methods such as addProperty, addChild and addInfo, but all of them are private.

It seems like a trivial change to make, but I can submit a PR if necessary.

[Altren]

Most resources have no support of runtime creation, but I have nothing against adding it