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

src/CMakeLists.txt

@@ -9,8 +9,8 @@ SET(LMMS_UIS "")
 SET(CMAKE_AUTOMOC ON)
 SET(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR})
 
-# Enable C++14
-SET(CMAKE_CXX_STANDARD 14)
+# Enable C++17
+SET(CMAKE_CXX_STANDARD 17)
 
 IF(LMMS_BUILD_APPLE)
 	SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++")

src/core/LinkedModelGroups.cpp

@@ -28,7 +28,6 @@
 #include <QDomElement>
 
 #include "AutomatableModel.h"
-#include "stdshims.h"
 
 
 

src/gui/widgets/ControlLayout.cpp

@@ -73,13 +73,12 @@
 
 #include "ControlLayout.h"
 
-#include "stdshims.h"
-
 #include <QWidget>
 #include <QLayoutItem>
 #include <QLineEdit>
 #include <QRect>
 #include <QString>
+#include <utility>
 
 constexpr const int ControlLayout::m_minWidth;
 
@@ -215,7 +214,7 @@ QSize ControlLayout::minimumSize() const
 	// get maximum height and width for all children.
 	// as Qt will later call heightForWidth, only the width here really matters
 	QSize size;
-	for (const QLayoutItem *item : as_const(m_itemMap))
+	for (const QLayoutItem *item : std::as_const(m_itemMap))
 	{
 		size = size.expandedTo(item->minimumSize());
 	}

src/gui/widgets/Knob.cpp

@@ -47,7 +47,6 @@
 #include "MainWindow.h"
 #include "ProjectJournal.h"
 #include "Song.h"
-#include "stdshims.h"
 #include "StringPairDrag.h"
 #include "TextFloat.h"