system: add path() to MarSystem and MarControl

New method return absolute path consisting of names only (no types).
system: enhance path API in MarSystem and MarControl

* New method 'path()' returns absolute path consisting of names only.

* Rename 'remoteChild' to 'remoteSystem'.

* Implement absolute path processing in remoteSystem and remoteControl.
inspector: display name-only paths on control and signal widgets
inspector: graph: beautify MarSystem name + type labels