Better describe relationship between AppD definitions and other manifests

[kriswest]

Area of Issue

[x] App Directory

Issue Description:

The use-cases for the App Directory Standard differ from that of app definition schemas, such as the Web Application Manifest, in that AppD is intended to provide access to and a means to discover details of multiple applications (some or all of which will exist on different domains to the directory itself), and can include details of how to launch them in multiple different environments. Whereas, a web application manifest is hosted on the same domain as the application, targets web browsers (which sometimes have different needs/form factors from containers or other environments that might implement a Desktop Agent) and are normally only referenced from documents on the same origin (see https://www.w3.org/TR/appmanifest/#start_url-member) allowing the application to be 'installed', once it has been discovered.

Since PR #437 (issue #314) was merged, the AppD's application definition can both specify the minimum level of detail need for launching a web app (via the details.url property) and can either embed or refer to (via URL) multiple other application manifests (via the hostManifests property) necessary for launching the application in proprietary containers and for launching native apps etc.. This relationship between the AppD's own definition format and directory and third party definition formats is not yet well described in the AppD's documentation, which should be improved to do so.

Additional Context:

• Introduce a general, host independent, fdc3.general.app.manifest manifest type describing an application #314
  • Update the Application schema #437