Style fixes to BSeparatorItem class.

Mostly meant for documentation with a few other style corrections as well.

Update copyright header.
Add BSeparatorItem class documentation
Style fixes to BMenuItem::GetContentSize and update docs

Rename width and height to _width and _height matching the header.

Update the documentation for this method as well.
BSeparatorItem: Style fix, rename archive to data

... missed it in a few places, trying to make variables names consistent.

Update hrev in docs
BSeparatorItem: horizontal, not vertical line