Look for qtsinglecoreapplication.h when Qt5Widgets module was not found,
and qtsingleapplication.h otherwise. Fixes#9196.
This also removes Qt4 support from the QtSingleApplication find module.
1. Use FeatureSummary module to show configuration results.
2. Invert option()/find_package() relationship: instead of
calling find_package(... REQUIRED) when option is set, rely on optional
find package call and PackageName_FOUND variable.
3. Refactor handling options that result in simple preprocessor defines
(actually copy the snippet from libtorrent) so that everything is done
in a single function call.
4. Populate target properties in order to get rid of
include_directories() calls.
