Enhance application to have a category, icon, URL and Read-Me fields

As a developer/deployer, I would like the application subsystem to allow categorizing applications into rough categories (Security, Traffic Steering, Monitoring, Drivers, Provider, Utility) so that I can easily discern the high-level purpose of the application.

I would also like to applications to be able to specify an icon, a URL link to the in-depth documentation for the app and a longer read-me description, so that I can learn more about the application.

Implementation Notes:

• Extend ApplicationDescription and Application to have category, url and readMe methods that return String values and an icon method that return byte[] value with image data.
• Enhance OnosAppMojo to extract onos.app.category, onos.app.url, onos.app.readme pom.xml property values into the respective ApplicationDescription fields; fallback on value of pom.xml description field
• Enhance OnosAppMojo to extract value for icon from the icon.png file in the OAR file; recommended size is 128x128.
• Value for description should be the first sentence (sans ".") of the onos.app.readme or description XML element.
• Value for readMe should be the entire contents of the onos.app.readme or description XML element.
• Enhance the CLI command to display the new data (sans icon)

• GUI enhancements are in a separate Jira ticket ONOS-3635.