Cnc: didn't solve the problem of having trouble at parsing of JSON during TinyG initialization

But added some more logging in case it happens.
.merlin: added B +threads
Cnc: ignore \000, apparently it sometimes appears on the stream on startup
Align.draw_overlay: render grid last
Widgets.level_store_widget: optionally store the level to an external variable
CncControl: instead of reference Z, provide a tool level

Compared to referenze Z the values are never reported as relative to
the level, but as absolute.