Let detect-toolchains be used like setup-qt.

Namely, let people specify a specific compiler and profile name
instead of offering only auto-detection.
Also rename to setup-toolchains for even more consistency.

Task-number: QBS-496
Change-Id: I8c144d7b4dd26e38652f10c1fff56af4daf3cecb
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>