Adds 'sidekick' type of generic content. This type allows rendering
generic content in the sidekick panel, in addition to the existing 'main'
type, which renders generic content over the presentation area.
Each generic sidekick content set through plugins is automatically
associated with a button in the navigation bar to toggle its panel.
Renames the generic component to generic content and adds the first
type, called 'main'. This commit is a preparation to allow generic
content from plugins to have types.
