Some build system improvements and cleanups
- cmake_minimum_required should come before any other line
- Actually use variables with the minimum versions
- Remove unused CMAKE_MODULE_PATHs
- Include KDECompilerSettings for common KDE flags passed to compilers
- KDECMakeSettings already enables CMAKE_AUTO{MOC,RCC}
- KDECMakeSettings already deals with enable_testing
- Port to ecm_add_test()