cmake: first port to Qt5/KF5

• bump the minimum cmake required to 2.8.12, as required by ECM
• find and use ECM, Qt5, and KF5, with the needed modules of each
• remove useless definitions and include dirs, now coming from Qt5::* and KF5::* targets
• switch away from the kde4_* macros, using native cmake commands, or KF5 functions