... to provide a set of urls indicating usable connection
points for 'server-using' tools (typically one, like gdbserver
and the Qml tooling, but two for mixed debugging).

Urls can describe local or tcp servers that are directly
accessible to the host tools, if needed port forwarding
could be set up when needed.

Use it as new base for GdbServerPortsGatherer for starters.

Note: Since none of the customization points for actual port
forwarding are currently provided by device implementations
only non-forwarding cases are working right now. Incidentally
this does not affect existing setups, as the only case where
it would be needed (Android/adb) have a complete custom
implementation. The medium-term plan there is of course to use
this new setup here and have the AndroidDevice implementation
only provide the forwarding, not the whole debugging (etc...)

Change-Id: I42c9783348cd430b1c435bbca56329c678ac485c
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>