Raising Qt requirement to Qt 5.4

[Ivan Čukić]

>> Is this documented in some wiki so we don't have to rely on people's memories
>> or someone digging up the corresponding email thread?
>
> Good point. Any suggestion on where to document this?

+1 since my gmail-search-foo was not up to the task - we have so many
threads regarding compiler and Qt versions, it is borderline funny.

It could go alongside compiler reqs:

https://community.kde.org/Frameworks/Policies#Frameworks_compiler_requirements_and_C.2B.2B11

Cheers,
Ivan