ComboBox: without a validator bindings to acceptableInput don't work

Bindings to ComboBox acceptableInput property don't seem to work at all if ComboBox has not been given a validator. This seems to be fixed by creating a seemingly unrelated binding to the ComboBox contentItem.acceptableInput property.

Steps to reproduce & fix the problem:

1. Run the attached example
2. => acceptableInput is logged as true BUT according to binding it's false
3. close the app
4. Uncomment line 45 in main.qml
5. Run the attached example
6. => acceptableInput is logged as true AND according to binding it's true