Clean-up requirements: cmake 3.0, qt 5.7, use min versions, deduplicate KF5

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• cmake 2.8.12 is really outdated, 3.0 is minimum in plasma & kf5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