The test is running pretty slow on debug builds, though (sorry). I see
no way how to make it faster while still exposing the issues with
std::accumulate().
Everything what was in src/ is now in src/Corrade, everything from
src/Plugins is now in src/MagnumPlugins, everything from external/ is in
src/MagnumExternal. Added new CMakeLists.txt file and updated the other
ones for the moves, no other change was made. If MAGNUM_BUILD_DEPRECATED
is set, everything compiles and installs like previously except for the
plugins, which are now in MagnumPlugins and not in Magnum/Plugins.
* Calling enable_testing() only in root path.
* Using CORRADE_CXX_FLAGS instead of our own set to make things easier
to maintain.
* Various cleanup and reorganization.