Fix submenu id assign

[mpotra]

This PR fixes an ID assignment bug for new submenus, reported in #6

The bug was that when determining the last used menu item ID, the algorithm did not search deep into the tree children, and for cases where a child id was greater than the last parent ID, the algorithm would only return the parent ID.
This resulted in new child id assignments to overlap with existing ids and thus send overlapping ids to D-Bus that would show previously assigned menu items in place of new ones.

Includes unit test for the fix.

[mpotra]

I've added telemetry-based debugging.
To enable, use the EXSNI_DEBUG environment variable, with comma-separated values as listed here: https://github.com/elixir-desktop/ex_sni/blob/fix_submenu_id_assign/lib/debugger.ex#L4-L13
Example:
EXSNI_DEBUG=all ./run to start ddrive with all telemetry events debugged.