On Plasma qqc2-desktop-style is used automatically, but not on other
DEs. Set it as a default and allow overriding it via environment
variables.
Also add it as a runtime dep so distros actually ship it.
Summary:
* cmake 2.8.12 is really outdated, 3.0 is minimum in plasma & kf5
* cmake_minimum_required should be at begin of toplevel CMakeLists.txt
* bump qt version to 5.7, matching the min Qt version of kf5 5.42
* with ecm being part of kf5 since early versions, share ${KF5_MIN_VERSION}
* use QT_MIN_VERSION & KF5_MIN_VERSION also for separate find_package calls
* deduplicate all KF5 components searched for in unconditionally included
subdirs
Test Plan: Still configures and builds with all options OFF & ON
Reviewers: #kde_connect, nicolasfella
Reviewed By: #kde_connect, nicolasfella
Subscribers: nicolasfella
Differential Revision: https://phabricator.kde.org/D11418
