qa: Avoid deprecation warnings for QList/QSet conversions

Qt 5.14 deprecated the explicit functions for converting between
QSet and QList, preferring instead the use of range-based ctors.
Unfortunately, those ctors were only added in Qt 5.14, so we can't
use them when compiling against older versions.

Add a util function for QList->QSet to keep the version check in
a single place. Replace the other direction by using QSet::values().
In some cases, conversions could be avoided altogether, or an STL
container be used easily, so do that.