Use static libraries for plugins which aren't used as dependencies (as
they don't export any symbols anyway), use shared libraries for plugins
used as dependencies (as they already export the symbols and static
libraries cause linker errors on mingw32).
* Explicitly installing header files instead of recursive glob (some
header are internal, thus no need to install them).
* Calling enable_testing() only in project root.
* Using MAGNUM_LIBRARIES instead of specifying all dependencies
explicitly.
* Using CORRADE_CXX_FLAGS instead of our own (heavily outdated) set.
* Removed FindMagnumPlugins.cmake, the module does essentially nothing.
* Various reorganization and cleanup.