[Boards Manager] Include core examples to Examples menu

Examples in core libraries are automatically added to the Examples menu. Unfortunately core examples are not.

Would like to suggest a sub-menu with the core example sketches and libraries instead of a single level. Could be named after the core.

E.g.
File>Examples>CORE>Examples>.... the examples directory
File>Examples>CORE>Libraries>... the examples in the libraries directory

where CORE could be, for instance, Cosa.

Cheers!

Start with the Boards Manager and the File > Examples menu described in the issue, then inspect how library examples are currently discovered. Done means core example sketches and library examples appear under a core-named submenu, with the requested Examples and Libraries sections.

Written by the indexing model from the issue text.