Move animation functions in demos to core pickle API

[solenya-group]

The slide and slideChildren APIs are go-to animation functions - they should be part of the core API.

Before adding them to the API, we need to round out the features and generalize appropriately. In particular:

The slideChildren functionality should co-ordinate nicely with another function for elements being attached/removed, e.g. to easily fade-in/fade-out.

Perhaps as a good proof of flexibility the tree sample could be animated too.

[solenya-group]

There's now a solenya-animations package. Samples have been updated accordingly.