Qt dependency policy

[David Faure]

Taking into account the feedback from the SUSE packagers
(about how dropping support for Qt 5.9 LTS the very month Qt 5.12 LTS comes 
out is a problem since not everyone can upgrade to Qt 5.12 LTS right away)
I have modified the policy to temporarily support 4 versions when the oldest 
(and newest) are LTS.

https://community.kde.org/Frameworks/Policies#Frameworks_Qt_requirements

-- 
David Faure, faure at kde.org, http://www.davidfaure.fr
Working on KDE Frameworks 5