Compile LMMS using C++17

This replaces `set(CMAKE_CXX_STANDARD 14)` by `set(CMAKE_CXX_STANDARD 17)`
wherever it is required.

Additionally:

* raise `CMAKE_MINIMUM_REQUIRED(VERSION ...)` to `3.8` (the minimum
  that supports C++17)
* `stdshims.h` is now unused and thus removed

include/stdshims.h

@@ -1,30 +0,0 @@
-//! Shims for std:: functions that aren't available in the current C++ versions
-//! we target.
-
-#ifndef STDSHIMS_H
-#define STDSHIMS_H
-
-#include <type_traits>
-#include <utility>
-
-#if (__cplusplus >= 201703L || _MSC_VER >= 1914)
-#ifndef _MSC_VER
-#warning "This part of this file should now be removed! The functions it provides are part of the C++17 standard."
-#endif
-using std::as_const;
-
-#else
-
-/// Shim for http://en.cppreference.com/w/cpp/utility/as_const
-template <typename T>
-constexpr typename std::add_const<T>::type& as_const(T& t) noexcept
-{
-    return t;
-}
-
-template <typename T>
-void as_const(const T&&) = delete;
-#endif
-
-#endif // include guard
-