SDKtool: Check for existing objects when creating kits

Check for existing Qt versions, toolchains and devices if
those ids are given.

Change-Id: I0447d55e01cc40e43669ba7999126b0260974c0b
Reviewed-by: Rainer Keller <rainer.keller@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>