Support arrays of structures in structured data (full setup)

[ralphlange]

The "full setup", i.e., using an opcuaItemRecord for the structure and simple records pointing to elements, only supports sub-structures and simple elements.

Extend the support to arrays of structures, so that one can use:

field (INP, "@item2 element=level1.level2[4].simple1")

to address the simple1 element of the 5th element of the array of structures level2 inside the top-level structure level1.

[ralphlange]

Will be done in the context of or after refactoring the classes for structure support (branch refactor/element-tree).