parametrize qqc2 version
Summary:
for styles in order to work at all they need to import
the latest available qqc2 version installed in the system
or any app using new qqc2 feature will fail to load.
do like in qqc2-desktop-style and set the import version
at build time
Test Plan: all generated files have 2.3 here as version and apps using qqc2 still oload
Reviewers: Plasma, bshah
Reviewed By: Plasma, bshah
Subscribers: plasma-devel, Frameworks
Tags: Plasma, Frameworks
Differential Revision: https://phabricator.kde.org/D10074