Update GenericItem docs

[jimtng]

I noticed several attributes aren't just read-only. They are actually read/write. For example, GenericItem#label= can be used to set the label.

Opening this issue to remind myself.

[ccutrer]

there are actually quite a few things that are read/write, but only documented as read. both core wrapper stuff, and dsl stuff (for example Timer#id is actually r/w, so that TimerManager#schedule can twiddle with the id of an already created timer). things like GenericItem#label= need investigated on if it's kosher to call that method (like if the item is from anything besides a manageditemprovider). GenericItem#state= is also a thing, but I'm pretty sure it bypasses the entire event infrastructure. I use it in Items::Builder, specifically for that reason.

[jimtng]

Currently I'd have to do Item.add_tag 'tagname' and Item.add_group_name 'anothergroup'. We could make it so this can be done by Item.tags += ['NewTag'] and Item.groups += [AnotherGroup]` and variations thereof.

Item.label= I believe translates to set_label.