setup: Allow to interactively choose {src,build} dirs.

Previously these could be changed but only by editing the resultant
kdesrc-buildrc. The first-run support that is coming along is a better
way to meet non-interactive or super-simple config needs so we can
afford to be more chatty in kdesrc-build-setup.

Differential Revision: https://phabricator.kde.org/D16757