Conform heading and breadcrumb rules to UI 3.0

[Curtis Hovey]

For IPrimaryContext, the default creation of a <h2> should not happen because it will not have breadcrumbs. The slot should honour the template if a heading is passed to it.

[Curtis Hovey (sinzui)]

Michael had these thoughts:
    So the heading slot always defaults to displaying <h2>context.title</h2>,
    with only the following two exceptions:

    1. if the current context is an IPrimaryContext, then the whole
    heading-slot+breadcrumbs will not display (as per sinzui's suggestion).

    2. if the current context is not an IPrimaryContext but we are viewing
    the index of the current context then it will default to an h1:
    <h1>context.title</h1>

    If and only if you need an inline-editable widget do you need to fill
    the heading slot - and in fact, we could call it something more obvious
    in this case 'inline_edit_heading_widget' so people are very aware when
    filling it.

[Barry Warsaw (barry)]

After extensive discussion with Martin, the headings and breadcrumb rules are defined here:

https://dev.launchpad.net/VersionThreeDotO/UI/Conversion#Heading%20rules