CMake and Kits: Fix interaction between them

On initial run we offer the user a choice between all the kits, which
are compatible with the cached generator. After the initial run, the
user can't change kits nor generators anymore.
Except if the builds into a new directory or adds a buildconfiguration,
then the user can choose between generators but not kits.

Task-number: QTCREATORBUG-7940
Task-number: QTCREATORBUG-7928

Change-Id: I9b663435cd2e021f7fe08379c1c487a6aebe8976
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Peter Kümmel <syntheticpp@gmx.net>