This call used to return a sorted list of BaseQtVersion *, which is
expensive as sorting might imply running qmake for all Qt versions.

This also makes BaseQtVersion::validVersions(...) return an unsorted list,
so go over all usages of validVersions(...) and use
QtVersionManager::sortVersions(...) where that is necessary.

Change-Id: I8f5dda20fc0819134c64f6a1ca32512500f71710
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>