JBIDE-16375: Added filter for early access

Based on the certificationId
JBIDE-16375: Refactoring

* Move inner type to own file
* DiscoveryViewer now extends viewer
* Improved filtering implementation
* License headers
JBIDE-16375: Added support for max version filter

* Improved parallelism on p2 resolver and general performances
* Refactoring to smoother UI (re-use instantiated items instead of
creating them over and over again)