[Development] Build system for Qt 6

[Adam Treat]

Lars gave a keynote saying pretty much the same. Simply is not true that 
we are planning major source compatible breakage for Qt6 so let's stop 
saying that.

On 10/30/2018 03:19 PM, Thiago Macieira wrote:
> On Tuesday, 30 October 2018 12:11:38 PDT NIkolai Marchenko wrote:
>>   >  That's not going to happen any more than our breaking source
>>
>> compatibility in
>> a major way.
>>
>> You are breaking source compatibility in a major way with Qt6 ... ;)
> No, we will break source compatibility in a minor way.
>