Add ability to define a "top level" category when mirroring repos using tycho-p2-extras-plugin

[andrew-tram]

A feature request based off of this - #4422

Currently when mirroring repos, using tycho-p2-extras-plugin:mirror, if available, the org.eclipse.equinox.p2.name from the respective repositories are used and displayed at the top level.

I'd like the option to define my own "top-level" category that then expands to the repos that are being mirrored.

Example:

• My Eclipse projects
  --- Buildship Extra
  --- Build Extras - Incubating
  --- Buildship: Eclipse Plugins for Gradle
  --- etc etc etc