Extend JSON API to request the list of profiles

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.