Line items in licenses provide additional information about a license,

which previously was limited to text strings with no icon when attached

to a license. Only RB.LicenseView subclasses could add more detailed

line items, and this required carefully formatting the HTML structure

for the row.
This change fleshes out the line items to make them more generally

useful. Now, LicenseInfo.line_items is a list of structured data that

contains content, an optional icon, and a flag indicating if the content

is HTML.
RB.LicenseView takes this and formats each line item as a detail row,

taking into account the icon and whether to show HTML.
License Providers are responsible for returning the line items for a

given LicenseInfo. By default, this returns the line items attached to

the license, but just like actions, they can customize this based on

other state (such as other licenses exposed by the provider).

Unit tests pass.
Made use of this in Power Pack.