Complete Implementation of ListOf

[lapp0]

ListOf field for types containing "list of" must be completed in widgets.py. It can behave similarly to AttributeSetOf.

[lapp0]

Currently elements of a list aren't branches in option_tree.py. there needs to be a way to access, for example, access networking.firewall.allowedTCPPorts[1] so it can be used as a URI and enable ListOf navigation and 'editing of list elements.

There are two ways to fix this:

• clean: have lib.nix retrieve the definition for list elements
• dirty: have option tree iterate over option_definition.obj to get the expression string and object for each subelement.
• less dirty, but suboptimal: retrieve elements in parser.py

Will try the clean solution first.