Custom elements can be configured to have user-defined properties, to behave in
their own special way, to interact with other elements, to change their
behaviour on certain conditions (by changing into other custom elements with
different properties) or to trigger actions that influence the game.

Each level has its own set of custom elements. To use the same custom elements
within one level set, use the "copy and paste" functions or use "save as
template" and "use template" to use the same custom elements from a single level
template for all levels of a set.
