Use ecm_configure_package_config_file

Ported with:
sed -i -e 's/\(include(\)CMake\(PackageConfigHelpers)\)/\1ECM\2/' \
       -e 's/\<\(configure_package_config_file\)/ecm_\1/' \
       CMakeLists.txt

See http://commits.kde.org/extra-cmake-modules/139f60d81653888f696ef433
Fix build

Missing include for i18n in grepoutputview.cpp, and override for a
method not actually in the parent class (which is now QQuickView rather
than QGraphicsView) in assistantpopup.cpp.