Add support for layout items

With this new item kind, users can describe layouts using basic building blocks.

Structurally, they behave similarly to folder, as they are inserted as non-leaf nodes in the items tree. On the front-end side, they are recursively rendered (similar to shortcut items).

A basic example is a columns layout item. The simplest form to implement such a layout is to simply add an column item type. In the first iteration, we can keep the same display as a folders in builder, and the number / order of children elements define the respective number / order of the columns. In player, we recursively display each column. Later, we may want to define a dedicated UI for columns in builder.

Other layout items may include grids, hidden, collapsible spoilers, ...

See #332 as well