delayed_helper::apply()
Clang to the rescue: - `void` isn't a literal type in C++11 - calling `f()` from `constexpr` isn't allowed in C++11 So, remove `constexpr` to simplify things.
DelayedCall
- Rename `delayedconnection.h` to `delayed.h` - Align `delayed_helper::apply()` with `std::apply()`