Fix creation of CMake config file, require libkomparediff2/ include prefix

• Use ecm_configure_package_config_file to generate cmake config file, with usual config file content + support for legacy variables LIBKOMPAREDIFF2_LIBRARIES & LIBKOMPAREDIFF2_INCLUDE_DIR
• Drop broken relative path for LIBKOMPAREDIFF2_INCLUDE_DIR: worked only for someone building libkomparediff2 & kompare in a fixed build dir setup, but nowhere else
• Require library consuments to use libkomparediff2/ path prefix, to add some namespacing to include paths KDevelop does this already in all released versions, Kompare has patch prepared to do the same (is also assumed to always use version of libkomparediff2 of the very same KDE Apps release)
• Drop LibKompareDiff2Import__ namespace from exported target name resulted in strange "LibKompareDiff2Import__komparediff2" target name, which also has not been seen in usage, instead all known users relied on LIBKOMPAREDIFF2_LIBRARIES variable in released versions
• Set "KompareDiff2" as exported target name
• Bump version to "5.1", so lib consumers can use this version to ensure this cleaned-up version of the CMake config file
• Set explicit KF5 min version and bump ECM min version to same using 5.16.0 for ensuring KDE_INSTALL_* variable names
• Set explicit Qt5 min version using 5.4.0 as limit set by KF5 5.16