It would be good to have an additional JSON API to query the all available profiles throught the IPC.

It is something like that the `qbs config --list` command does.

For the IPC we can introduce a new request `type`, like `get-profiles`. The request content can contains the `settings-directory` property to specify a concrete settings location.

This would be useful for the some IDE's integration to request the QBS profiles. Because right now, e.g. the VSCode extension calls the separate `qbs config --list` command and parses its output to show the list of profiles for the user.