AbiWidget: Add method to check whether a ABI is custom or not

Change-Id: I893f06d5863573a6b787b65d02262bbfc6eee2f7
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
GccToolChainConfigWidget: Keep custom ABIs

Do not override a custom ABI when a new set of supported ABIs is
detected.

Task-number: QTCREATORBUG-10582

Change-Id: I8476b520e80f39037240175cac135ad68752f725
Reviewed-by: Daniel Teske <daniel.teske@digia.com>