* cmake 2.8.12 is really outdated, 3.0 is minimum in plasma & kf5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