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

tests/CMakeLists.txt

@@ -4,7 +4,7 @@ INCLUDE_DIRECTORIES("${CMAKE_SOURCE_DIR}/include")
 INCLUDE_DIRECTORIES("${CMAKE_BINARY_DIR}")
 INCLUDE_DIRECTORIES("${CMAKE_BINARY_DIR}/src")
 
-SET(CMAKE_CXX_STANDARD 14)
+SET(CMAKE_CXX_STANDARD 17)
 
 SET(CMAKE_AUTOMOC ON)